I'm in more-or-less the same boat, so I sympathize. There are nooks and crannies in every big city where you could have a semblance of a shop, but you're right. It's hard. Especially when you're not sure how long you're going to be around, so it's tough to justify investing.

That said, it is amazing how much some people do with so little. Look at Clickspring's work (http://www.clickspringprojects.com/). The man is a freaking virtuoso and I think his shop is 4 feet x 10.