The latest edition of the Apogee Newsletter (Issue #317) describes how to make screw-on 18mm retainers from pint sized water bottles. The article, written by Chris Michielssen (master modeler, owner of Odd'l Rockets, and blogger at Model Rocket Building), provides a detailed explanation of the process. These retainers will work on models based on BT-55 or larger and will require an engine block. He didn't address using a masking tape thrust ring, but that might work. I think I'll try this on my next 18mm build.
Update: Chris confirmed that tape thrust rings won't fit in the screw-on cap. That means the 18mm RMS case won't fit either. So, while I still want to try this, I have to wait for the right build. Since I don't build a bunch of 18mm stuff, I just hope I remember this tip when the opportunity presents itself.