The reason that this project is so easy is that it uses old computer speakers as its base. Computer speakers already have the circuitry built in, so no extra effort or amplifiers are necessary. I got a very nice pair for $5 off of Craigslist (they're normally $15).
-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.