Templating can be a bit of art work and/or craftsmanship, but it doesn't have to be in this case. You can use latex or natural rubber and make a precise fit mold
of the inside of the ring. Of course you'll need to apply a release agent and my usual choice is automotive wax, liberally applied in a few coats. You could also use epoxy
or polyester resin with this technique. Brush on the mold
making goo, apply some reinforcement so it'll hold it's shape when removed, such as fabric
, cardboard, bits of wood, etc. and make sure the reinforcement is also well bonded to the mold. Even concrete could be used and if you did, use "thin set" and cast some handles in the back of it so you can pull it out of the ring.
I'd use epoxy
, just because I have it handy and I'd apply a few straight coats to build film thickness on the rig, then I's apply thickened epoxy to get the corners and recesses, then the reinforcement, handle, etc..
Naturally, you have to protect areas that will receive the epoxy. You don't want it running down into some place, but this is fairly easy with packaging tape.
With modeling clay, I'd be afraid it would distort as it was removed, though I know a few that swear by it. Cutting templates from Mylar or drafting velum will remain stable and you can see through it, which could be handy.
A joggle or tick stick is such a confined space is useless, even though I use these a lot, sometimes they just don't work.