Step 1: Collect Supplies
Two 4X4 lego bricks. You can use the same color or two different colors.
One 4X4 lego plate. Again, you pick the colors.
One 4X4 lego coupling plate - This is lego design ID 3176 if you are on the pick a brick lego website. If you do not care if this goes on a keychain then you can just use another 4X4 plate.
Razor blade/hobby knife
Marker of some sort (I decided against permanent)
USB flash drive that you do not mind breaking apart
Needle nose pliers
Not necessary but useful:
Drill with small drill bit
A dremel may also be useful but I could not find mine
Step 2: Disassemble Flash Drive
Place this section into your computer and mark where it stops in the computer so that you can make sure to leave at least that much out of the brick itself. Mine didn't end up being a problem either way but I checked just to make sure. If you're using a different brick, it may matter.
Step 3: Prepare the Bricks
Use your diagonal cutters and needle nose pliers to remove the cylinder out of the middle of the bricks. I did this by using the cutters to cut 4+ vertical lines down the cylinder and then using the needle nose pliers to twist the sections out. This worked fairly quickly without much effort.
Place the 4X4 brick that you want to house the USB onto the 4X4 plate with the hole in it.
Place the USB on top of the other 4X4 plate. You want to make sure that the USB sits on top or higher so that you do not have to shave down any of the pegs on the plate. Use this to mark where your hole will be on the main brick. I used a zig pen so that I could wipe it off easily afterward. You could also use a sharpie. I believe it can be removed with alcohol on many surfaces but I decided not to try on this one so I'm unaware of it's effect on legos.
Cut out the hole.
After marking the rectangle, I used a razor blade to make the lines then wiped away the marker . I think drilled 3-4 small holes in the lego within that rectangle so that I could fit the tip of a razor blade in them. I then began shaving and cutting away at the plastic to make the rectangular hole so that the USB could fit snugly. I checked often to see when the heights and lengths were right. Mine was a little too tall but since I was epoxying that side in I didn't worry about it too much.
Step 4: Set the USB
I also made sure that the peg side of the block was up when it was inserted.
I used epoxy. There may be a better product that I'm unaware of. I tried to use as little as possible as not to mess with the USB itself. I pulled the USB out a little (though not all of the way out of the hole) and used a toothpick to place a small amount of epoxy on the "back" of the USB then pressed it into the wall of the brick where it was supposed to sit. Then I placed a small amount of it on the bottom edges of the brick and pressed the plate into place. You may have a little come out at this point. You can use a paper towel or any other disposable cloth-like product to wipe away any excess. If you wipe away immediately then you won't have to sand it off later and you can keep your clean lego look. I had a 5 minute setting epoxy so it didn't take long. If you want to epoxy the plate onto the "cap" side of the drive then make sure both bricks are ready to epoxy together so that you do not end up wasting the self stir tip of the epoxy (if you are using self mix epoxy). I did decide to glue my cap on (see next step) so I had to work a little fast on the second part before realizing that I could not reuse the tip since all of the epoxy would have already been mixed and set inside the applicator tip. I found out that if I took the tip off I could mix just a little bit from both sides and it would be fine but this is just a tip from my mistake!
Step 5: Make the Cap
I still epoxied the plate to the brick after I was done trimming.
If for some reason you do make the hole too big. You could always add a 2X1 plate on top to make a "snap" for the lid or another brick of another sort. I was really hoping not to have to do that and I was pleased with my outcome.
Check often and make small adjustments are my only tips for this. Also, make sure you put the plate onto the brick before lining it all up.
Step 6: Finish
If you'd like, you can place a split ring through the hole to give to a friend or to snap on your own keyring.
Snap the cap on and show off your new lego keychain USB drive!