The keys that I have used could handle the weight of any Hot Wheels car that I have seen. By the time you mount it and hot glue it all together the USB port doesn't even wiggle.
I created it a while back to send to a friend as a video. Haven't had time to recreate as a slide show. I figured I wait and see how the feedback was before I spent the time. ;-)
BTW, these are all 4GB USB keys. I have a 16GB that will fit in my next one. Also, make sure that you pay attention to how the key fits. If it shorts out on the diecast metal it will fry your key. A friend of mine learned that the hard way. ;-)
http://i263.photobucket.com/albums/ii143/oxjaisunxo/DSC04520.jpg