How about making a sleeve to go inside your ring? 1.5” PVC Pipe has a 38mm inside diameter.
Hmmm... good idea. Not visually ideal, but it has definite labour saving possibilities. A local DIY supplier does 40mm black PVC pipe which presumably would have 2mm-ish walls. Might have to buy their shortest length and do some experimenting.
Nature and nature's laws lay hid in night;
God said "Let Newton be!" and all was light.