To decipher what I'm going to write next:
Turret Cap = the part you turn with the MOA writing on it.
Turret Cover = the part that threads on over top of everything so you don't see the cap.
I tried some copper tubing that wasn't quite right, the wall was too thin and the turret cap slipped "under" the copper tubing and didn't "stop" it like it needs to. I have some carbon-fiber tubing w greater wall thickness coming that I hope will work better. I'm just measuring the gap between the turret cap and the top of the threads for the turret cover then using a roller-style tubing cutter.