The Ruby Amp is a great little battery powered solidstate practice amp. The design is based on the Little Gem design which in turn comes from the Smokey Amp designed by Dave Stork on www.blueguitar.org. The design uses relatively few components and is a good DIY project. Cigar box guitar maker J.K. built a Ruby Amp but found himself hunting for a suitable enclosure for it. Then he noticed an old magic 8 ball that looked just right with a perfect sized hole for the speaker.
Is this a good amp? ...<Shakes ball vigourously>... Signs point to yes!
This is a sample project from MAKE magazine, which shows you how to build a cracker box guitar amplifier for around $5 . It only shows the first few pages of the project, but it gives the schematic for the circuit and you should be able to work out how to finish it yourself. For the full article you will need to buy issue 9 of MAKE. Or why not subscribe: it's an excellent magazine for would be inventors and often features music related projects.