If you are like me and only run one 18650 at a time in your Picocalc, then you should try my storage cell. 

A simple place holder that occupies the second cell position in the battery compartment. As I like to have the original Clockwork allen key with me, I was merely taping it to the battery door, but here is a better solution. 

There are 3 versions. One is just for the allen key. A second includes holes to store extra M3 screws x8. The pictured version also includes 3 slots for micro SD cards.

Easy Print, no supports needed.

Note: these are designed to slot in at a 45 degree angle (as pictured) so the allen key does not make contact with the PCB.