ICOM IC-A20 bottom cap

Bottom cover for the ICOM IC-A20 for use without a battery.
Two versions provided:

- `ica20-cap-supports.stl`: version with hand-crafted supports
- `ica20-cap-nosupp.stl`: no built-in supports

The cap slides and clicks without play. Use a 0.4 nozzle with 0.2 layers (or lower).
I recommend to print the version with hand-crafted supports for a better results.
The supports can be easily removed with a knife.