Hi Laura.
When you don't have a retaining ring, a few drops of rubber cement works well if the hole is approximately the correct size. I use a 23C and a 45MX. Both use the same lens board (4X4). I have a number of 1/8" solid aluminum blank boards. If you measure the diameter of the hole necessary (measure it exactly with a dial caliper), I will lathe bore a hole in a board and mail it to you FREE. How's that for a deal? I happen to have a metal lathe and can machine just about anything.
PM me with the correct diameter of hole, and a shipping address, and I'll send you the board. It may need a little edge filing to fit as I had a number of the boards cut on a large press, so the size dimensions of the boards are not exact. Having said that, the last two boards I made up for my own use only required minimal filing to fit in the enlarger.
The board will need to be painted flat black before you glue the lens to it. The rubber cement is easy to remove if and when you ever need to.
Rick Jason.