There are a couple of great matchbox/hotwheels flash drive instructables, but as things get smaller so must our pointless case mods!
Step 1: Gather Materials and Disassemble Car
I got a bunch of cars in a lot on ebay for 5$. I bought the 4gig flash drive on ebay from a guy in Hong Kong for $3 (brand new!).
Some of the cars have screws and some of them have pins or rivets. The one I chose had the latter, so I just carefully pried the bottom off with a flathead screwdriver.