P.S. I understand that an FM transmitter is a viable solution. It has been pointed out several times in the comments. However, this is what I made, and it is merely one way of doing things. Thanks.
Step 1: Materials
-Computer speakers - $5 (on Craigslist)
-Sheet plastic - $5
-Car power adapter* - $9.99 (at radioshack )
-Stereo plug - $3.99 (for a pack of two at radioshack )
-Window screen - $0 (I got mine from an old window)
-Small nuts and bolts (I had plenty of these already)
-Hole-cutting drill bit
*A word about adapters: my speakers ran on an AC adapter that produced 12V DC at about 1.5 amps, so I got a 12V 2A adapter. There are other adapters with different outputs, so be sure to get one that fits your speakers.