I built this slide guitar using a license plate for a resonator top, reclaimed oak, and broken ukulele parts. All totalled it cost me zero dollars and looks and sounds great. You can buy equivalents for any of the materials I had found, or scavenge them for yourself. It's a fun build that results in a unique sounding and useful instrument!

Step 1: Gather Materials

I used 1inch by 2 inch oak that i reclaimed from an old church organ. Any hard wood would work well, pine will work also, but it might not sound as great or last as long. Get enough to wrap around the edges of your license plate and about 29 inches for the neck, plus a couple feet extra, just in case.
Get an old, American/Canadian license plate. It doesn't matter which state, but make sure it's a standard size and not European or for a motorcycle or something.
a hinge as wide as your wood pieces, with 3 holes in each side
one bolt about the same length as the long side of your wood pieces. Something with coarser threads and a shaft under 1/4 inch works better
one bolt a bit longer than the widest part of your wood pieces, with a nut that fits it
3 guitar strings
1 1/2 in screws. 1 1/4 work also but aren't as good - at least 12 of them
1/2 inch broad headed screws. the type that look like they have a washer attached to them. alternatively you can get regular screws and use small washers that fit them. You'll need 4 of these
3 tuning pegs. I got mine from a broken toy ukulele, you can order them from guitar parts stores online.
a drill bit a little wider than the posts on your tuning pegs
a drill bit a little smaller than your screw shafts
a drill bit as wide as you screw heads
a drill
screwdrivers or driver bits that match your screws
wood glue
clamps
something to shape wood with. It can be a palm sander, sand paper, a belt sander, a rasp, or anything that will round over sharp edges. I used an oscillating belt sander because, well, I had one.
a saw. I used a standard chop saw, nothing too fancy, but a hand saw would work as well.
jigsaw or coping saw

Step 2: Measure and Cut the Frame

Using the license plate as a guide, measure and mark your stock so that when assembled, the outside of the box frame will be flush with the edges of the license plate. Don't worry if the sharp corners stick out, we will round those over later.

Step 3: Pre Drill, Countersink, and Screw Together Your Frame Pieces.

Use a drill bit slightly smaller than the shaft of your screws to pre drill two holes at the each edge of your long frame pieces where they will connect to the edges of the smaller pieces.
Then "countersink" (use the bit as large as the screw heads to widen the very top of the hole to a very shallow depth) the holes so that the screw heads sit flush or below the surface.
screw the frame together using this method

Step 4: Mark the Frame Where the Neck Will Go

This is a "through-neck" guitar, which means the neck piece will travel all the way through the body (frame) so here we

measure to the center or the "top" of the frame (where the neck will stick out)
mark around the neck against what will be the "front" of the guitar body (as pictured)

Step 5: Cut Out the Slot for the Neck

use a jigsaw or coping saw to cut out the slot where the neck will sit

Step 6: Fit the Neck Against the Back and Affix

place and make sure the neck is sitting flat against the bottom of the frame
trace the bottom of the neck where it meets the frame
drill 2 holes for the screws in the bottom of the frame where it meets the bottom of the neck, inside the mark you made.
countersink the holes on the outside of the frame
drive screws through the holes and into the bottom of the neck

Step 7: Drill and Affix the Neck to the Top of the Frame

drill and countersink 2 holes through the back of the neck where it meets the top of the frame
drive screws into the holes to affix the top of the neck

Step 8: Creating the Headstock

For the headstock of the guitar, I took one short cutoff from the frame (about 2 inches), and one slightly longer cutoff (about 3 inches). You can custom cut new pieces of your wood to these lengths if you don't have any cutoffs already laying around. Here's how i made them into a headstock:

take the longer piece and cut it in half lengthwise, so that you have 2, 1 inch pieces, 3 inches long
place the 3 inch pieces on either side of the two inch piece so that all of the tops line up together
glue and clamp them in place
make sure they fit snugly over the top of the neck

Step 9: Mounting the Headstock

Once the glue on the headstock has dried:

spread glue on the sides of the guitar neck, from the top about one inch down
spread glue on the inside of the headstock slot
slide them together and, with the front of the guitar facing down, raise the end of the headstock about a half an inch (to give it an angle of a few degrees)
clamp the ends of the slot around the sides of the neck
after that glue-up dried, you can use wood glue and sawdust or even small scraps to fill any gaps on the front, where the neck meets the headstock. This is mostly for looks, but it does look much nicer.

Note: I left the clamp from my first glue up of the headstock attached for these steps just to be sure I didn't mess it all up. If your glue is completely cured then you wont need to worry about this. if you want to do it, it won't hurt.

Step 10: Mount the License Plate

place the license plate on the "front" of the guitar body frame
pre-drill holes where the mounting holes are in the plate
drive your broad-headed screws into the holes

Step 11: Shaping and Smoothing Your Work

Use your rasp, sander, or sandpaper to:

round over the edges on the frame to match the ends of the license plate
sand the joint where the neck meets the headstock
round over the edges on the back of the neck
round over or smooth any other edges you may want to on the guitar

Step 12: Drill and Mount the Tuning Pegs

drill your holes for the tuning pegs far enough apart so they wont bump up against each other
place the pegs through the holes and pre drill the holes for the mounting screws
drive in the mounting screws

Step 13: Paint Job and Clear Coat

Both of these steps are optional, but after you clean off the dust on the guitar you can give it a custom paint job and protect it with a clear coat of lacquer or polyurethane. Be sure to unmount your tuning pegs first!

Step 14: Drill and Mount the Hinge

place the hinge as shown
pre drill the holes for the mounting screws
drive the mounting screws

Step 15: Stringing the Guitar, Adding the Bridge and Nut

Use the three holes on the hinge to loop the bottom of the strings through
string them up to the tuner pegs and tighten them most of the way
add the bolt without a nut as pictured where the headstock meets the neck
add the bolt with the nut on the body, under the strings, 24 inches from the bolt at the top
tighten the strings to tune

Step 16: Enjoy Your New Instrument!

you can use a short steel socket as a slide if you don't have one
there are many great resources online about how to tune, play, and customize your instrument
have fun making music!