I saw one of these in the spy museum in DC and thought it was pretty awesome.
Recently, I discovered you can buy these, and the commercial version probably holds
alot more then mine, but it is fun to make your own cold war spy gear.
This might not work very well if you carry it around in your briefcase or something,
but in the proper place the bolt is probably one of the best hiding places ever.
You only need a few things for this
Two bolts, one big one small
Tap and Die set
High strength putty epoxy (PC7 works great).
It would be really cool if someone with more know how then I would make one of these that was a flash drive.