Interesting. To make up a dummy round, you could insert a wood block inside the shell case,mark it at the mouth of the case, then measure on the shell how far you wish to insert it. Measure down from your mark the distance you need to insert it, make a line, then cut it off. This will allow the shell to go in, and not go in fully.

To resize it, measure the diameter of the shell, and the thickness of the mouth. Now comes the tough (read $$$ part) unless you have a friend with a metal lathe. You will have to make up a mandrel for the inside diameter, less a small amount. Then you will have to make up a split collar that is the diameter of the shell plus TWICE the mouth thickness LESS a small amount for holding.

If you have a friend with a wood lathe, find a piece of hardwood and try the above procedure. It will probably work for a one-off deal.

Next, insert the mandrel inside the case, put the split collar around the outside of the case, and clamp the collar together. Try the shell in the case. If not tight enough, you might have to take a bit more off the mandrel and reduce the collar diameter a bit to squeeze it down more.

That is what I would try. Good luck.