It is made from a 1/4” bolt. It can't hold anything much bigger than a postage stamp. The general design could easily be used with a larger bolt. A 5/8” bolt would be a good size for urban geocaching or would be about the right size to conceal a micro SD card. An O-ring could be added to the design to make it water tight.
A lathe is required for this project but it is a good chance to practice some basic skills. Total cost for materials was less than a dollar (I think actually free) and it took about an hour to make. Unfortunately I didn’t take any pictures when I was building it so I’ll do my best to describe how it was built.
I used a 1/4” bolt about 2" long but any bolt or even a large spike or nail will do. An old used bolt would be good if you wanted it to blend in with its surroundings.
Clamp the bolt in the 3 jaw chuck with the head sticking out about 3/4”. Using a hack saw cut the head off the bolt leaving about a 1/2” of the shank on the head. This could be done with a parting tool but a hack saw was faster for me.
Face the end of the piece remaining in the lathe. Leaving the corner sharp will make it looks more seamless when assembled. With a #25 drill, drill a hole about 1 1/4" deep down the center of the bolt to form the cavity. The hole is then taped #10-24 to a depth of about 3/8”. It is important that the threaded hole is square to the face or the head will not sit square. De-burr and remove from lathe.
The head is then clamped in the 3 jaw with the shank sticking out. Turn down the shank to 0.15” by about 1/4" long. Small cuts are required since it is only being held by the head. A small grove is added just below the head to ensure that it seats fully when assembled. The shank is then threaded using the lathe. The threads were then chased with a die.
Everything is then cleaned up and assembled. The finished bolt can hold a piece of paper about 1" square.