Note: this post is from 2007, outbound links may be broken.
Make a cassette loop for about 6 seconds
Instructables has instructions on how to make a tape cassette that loops.
6 second cassette tape loop instructions
Looks like it might work!
I just dusted off an old tapedeck I was going to sell and decided against it (since it didn’t look like it’d make more than 10 bucks or something) so I will try to find some time to make one of these cassette loops.