Centering Rings - DIY! Part 1

I need some "non-standard" centering rings for an upcoming build.
A series of three interchangeable engine mounts will be made for a Semroc Centurion.
I don't use BT-20 tubes for 18mm engines, they are too thin and weak. ST-7 tubes will be used.
No BT-50 tubes for the 24mm mount, again too thin. I have some thick walled BT-50 tubing.
The Semroc Centurion uses a Centuri style ST-16 mainframe.
Sure, I could put in a custom order, but sometimes it's quicker and cheaper to just make your own from scrap framing mats.

I don't have a circle cutting compass. I know, I should have one. These rings will be cut with a new #11 X-Acto blade.

Trace around the larger diameter tube first. You won't be able to visually center the large tube if the smaller motor mount tube circle was drawn first.
Use a sharp, thin mechanical pencil.

With the larger tube diameter drawn you can center the inside tube circle inside of it.
Draw and check the center circle. If it's off, erase and redraw.
It might take a few tries.

TIP: Cut the smaller inside circle first!
You'll want to have plenty of border material to hang onto.

Here I'm cutting a little inside the pencil line.
The traced line was drawn around the outside of the tube. You have to make allowances for the thickness of the pencil line next to the outside diameter of the motor mount tube. You can always sand to a perfect friction fit.