I'd highly recommend roadway signs for practice. Erm, at least something just like them... Most aluminum cases are made with ~1/16th inch thick aluminum, and that stuff is easy to get in small sizes. I'd not suggest full-scale practice, since big aluminum sheets can be pricey and relatively hard to get.
The other advice I'll give is that make sure you'll like the casemod in the future, since not every fasion endures for long. Also, always make sure that if you mess up, you can either re-order that part or live with the blemish. Otherwise you'll regret doing it forever and not be able to enjoy the rest of the casemods. As for practcing with the dremil, I'd suggest that you try as many 'high-risk' manuvers as you can think of, since it's always that one certain cut/grind you didn't expect that screws you & your project over.