In this hot summer days, your modem may be get crashed in cause of high temperatures. and high temperatures can make your modem work slower. just a small fan can make your modem cool enough. cause you just need to supply air circulation.
here's the things we'll need:

-cutting blade
-soldering iron
-a piece of soldering wire
-knife
-nail cutter
-double-sided adheshive tape
-a computer fan of your choice
note: the fan should use the same voltage with the modem, and the modem+fan power(generally in mA unit) shouldn't exceed power adaptors power. example, my modem wants 800 mA, and fan needs 170 mA. my modems adaptor can give up to 1000mA realtime. this is okay.
-your modem, of course.

Step 1: Cutting the Fan Air Hole

in this step, you need to decide to where will you open the hole.
after this, heat your soldering iron, and open the hole. then fine edges with nail cutter.

Step 2: Installing the Double Side Tape to Mount the Fan

first, place a piece of tape under the fan(air out side).
thenshape it with cutting blade.

note: if you want to mount your fan with screws, you wont need this step.