here's the things we'll need:
-a piece of soldering wire
-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
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
thenshape it with cutting blade.
note: if you want to mount your fan with screws, you wont need this step.