If you disassemble the assembly of a small refrigerator, you will find a piston like the one shown here. Given their size, they are ideal for making key rings. As normally you do not have access to these elements, I attach a copy for 3D printing.

I suggest printing the whole assembly with a height of 0.15 mm, a infill of 20 %, using a raft so that the assembly is well seated on the base.