Tunable caps for a marimba's resonator

I need help designing three variations of a disc that will act as a stop in a aluminum tube for a marimba.

two discs for each diameter with a mild variation in diameter
Disc A -------- Disc B
3 and 5/16 --------- 3 and 3/8
2 and 5/16 --------- 2 and 3/8
1 and 9/16 --------- 1 and 5/8

the discs need to be 1/2 inch thick

Disc A needs a hole that is threaded 1/2"-13(threads per an inch) all the way through

Disc B needs a similar hold that only goes 3/4 of the way through the disc and has a flat bottom.

When stacked these holes need to line up.

I’m planning on using PLA as these will not support weight

any help is appreciated even if some direction in terms of what software to use that is simple to accomplish this task, I am a woodworker and welder and need this dumbed down for me. I don’t mind working in metric if that is what is needed for these type of machines.