Portable Bluetooth speakers are great, but they can be costly. If you want to make your own, Instructables user bartoosen shows you how to make your own for about $US14.
What's great about bartoosen's build is the fact it doesn't really look a DIY project, which is surprising because it's housed inside of a piece of PVC pipe. Beyond that, you'll just need a Bluetooth module, various cables, a small amp, speakers and a battery. It's a pretty easy build overall. Head over to Instructables for the full guide.
Portable Bluetooth Speaker 2x3W [Instructables]