I have used a razor saw with pretty good results. If you want to make thrust rings, remember that you can peel a few layers off the inside to make it a teeny bit lighter (works best to do this after you cut it from the rest of the case).
Also, sometimes a 1/4 inch length of motor case comes out kind of limp and mis-shapen after being cut. Mush it back into the proper shape, roll the inside surface with a dowel to press the paper back together, and use a few drops of CA to 'rebuild' the ring.
One more- because the motor case nominal OD is so much smaller than the ID of BT20 tubing, I usually cut a narrow strip of paper to glue around the outside of the case. It doesn't take much and you can get a good snug fit with your new thrust rings.