You want a good snug fit.
Find a USB key that looks easy to disassemble. They all are a little different so look around. for the right one.
Carefully open the case to reveal the circuit board.
This is what you are after.
Cut the wood pieces from the pattern included in the pattern book. And lay the circuit board on the cut out and measure how deep and wide the hole need to be to insert the key.
Set up a drill press to drill holes to the required width and depth for the circuit board. I used a 1/4" drill bit. Measure the thickness of your circuit board to make sure that's the bit you need.
Here are all the final pieces ready for assembly.
You want the circuit board to fit all the way down to cover the whole board. Just leave the connector sticking out. You want a snug fit but if it's loose use hot melt glue in the hole before you slide in the circuit board.
Here is the USB stick in it's base ready to use.
Insert the stick and you are ready to save your valuable data for safe recovery or transportation. You can get 16gb USB sticks now for under $30 if you look around and the small 2gb keys are almost giveaway items anymore.