This is a basic Double Cat tunnel. I have 2 large cats and they love the tunnel that comes with cat trees. Problem is the cats are too long for them and are always hanging out an end.
I found many ideas on how to make cat tunnels, most were attached to a cat tree. I wanted something independent and sturdy enough to hold up under their play. She weighs 14 lbs and he weighs 11 lbs (kitten). They have tested it well and have yet to knock it over.

Materials
(3) 1” x 10’ 450 Schedule 40 PVC Pipe
(8) 1" 3-Way PVC Elbow Fitting
(8) 1" 4-Way PVC Tee Fitting
(2) 1" 5-Way Cross PVC Fitting
(1) 12" x 48" Heavy-duty Cardboard Tube for Concrete
Remnant Carpet (minimum 4' x 10')

Tools needed
PVC Cutting Tool
Rubber Mallet
Tape Measure
Crayon or Marker
Utility Knife
Handsaw
Carpet Knife
Liquid Nails Adhesive (or similar product)

Step 1: Measure and Cut PVC

We used a crayon and marker to mark the PVC and lined up the marks with the PVC cutting tool.
(3) 1” x 10’ PVC Pipe cut to:
(12) 1” PVC Pipe cut to 9”
(21) 1” PVC Pipe cut to 12”

Step 2: Cut Tube

We cut the tube in half making (2) 24" tunnels. We measured out 24" marked it on the Cardboard Tube and cut with a utility knife. You can also use a handsaw

Step 3: Cover the Tunnels

We visited the rug area at Home Depot and purchased a Remnant Carpet. You want the carpet to be minimum 4' x 10'. You are covering (2) tunnels that are 24" long inside and out. If you do not want to cover the inside, you can just use a 4' x 2' section to cover the outside of the tube.

We chose a level loop carpet. I wanted something that I would not have to take apart and clean all the time. The level loops do not attract and hold onto the cat hair like cut pile does. The level loops the cats can still scratch but they seem to prefer a cut pile.

You want to start with a remnant that is at least 4' x 5' for each tunnel. Place the tube in the middle of the remnant, so that the 4' sections wraps around it. You want at least 1' of carpet on each end of the tube. The 1' sections will be folded into the tube to cover it.
I wrap the carpet around the tube and make sure that it completely covers the circumference of the tube. Make several marks along the carpet, 1" more than where it overlaps. Trim the carpet, so that it covers the tube completely with a slight overlap.
Apply Liquid Nails to the back side of the carpet only where the carpet backing touches the tube. I start along one end of the carpet and apply liquid nails rolling the tube and applying liquid nails stretching the carpet along the tube till it meets the other side. I then trim any excess carpet and glue the seams together. Let it sit for a few hours.
It may be easier if you place the tube on a table to do the next step. Find where the two seams meet, measure about 6" and cut the carpet into a strip, see picture. Apply Liquid nails to the strip and stretch and pull the carpet into the tube. Cut another strip and apply Liquid nails and stretch and pull the carpet into the tube, line up the edges with the first strip that you glued. Keep going till you get to the last strip. This strip you will have to trim along the edges to get it to fit next to the first and last strip you glued. Flip the tube over and do the same to the other side. you want the carpet to meet in the middle. If there are any gaps you can take remnant carpet and glue to the inside.

Step 4: To Cover or Not to Cover, That Is Your Cat's Preference

We covered one with rug and left the other as is. Our cat liked both options. In the end we covered both since they like to scratch and it kept the tubes from shifting.

Step 5: Assemble Bottom & Top Layer

For the bottom and top Layers you will need:
(4) 1" 3-Way PVC Elbow Fitting
(2) 1" 4-Way PVC Tee Fitting
(4) 1” PVC Pipe cut to 9”
(3) 1” PVC Pipe cut to 12”
For each layer

Connect the PVC pipes and Fittings. See photo for correct orientation of Fittings and location of PVC pipe sizes.

Step 6: Assemble Middle Layer

For the middle layer you will need:
(4) 1" 4-Way PVC Tee Fitting
(2) 1" 5-Way PVC Elbow Fitting
(4) 1” PVC Pipe cut to 9”
(3) 1” PVC Pipe cut to 12”
Connect the PVC pipes and Fittings. See photo for correct orientation of Fittings and location of PVC pipe sizes.

Step 7: Build the Unit

Attach (6) 12” PVC Pipe to top of the 3-Way PVC Elbow Fittings on the bottom layer (see photo)

Step 8: Build the Unit

Place the Middle layer on top of the (6) 12” PVC Pipes from the Bottom layer. (see photo)
Slide tunnel between layers. You can also put the tube in between the PVC before placing middle layer on.

Step 9: Build the Unit

Attach (6) 12” PVC Pipe to top of the Middle layer. (see photo).
Place tube in between PVC pipes.
Place the Top Layer on top of the (6) 12” PVC Pipes from the Middle layer.