Hi Mantares
What you are describing is exactly the same experience I had using the Mantua provided rings: impossible to make them fit to the proper diameter, laser cut burn hard to sand, and very fragile.
I gave it a first try in using metallic rings. this is what I am using
https://micro-modele.fr/fr/recherche?controller=search&s=plat+de+laiton
Sorry it's in French but quite easy to understand. I am using the 1.5mm x 0.5mm size. I tried the 1mm thick but it was quite hard to make a nice ring.
I gave it a few try and it works quite well. As I previously said I am not at home so I have to do with very few tool. I tried to close the ring with CA glue but it's not really strong enough. I'll have to solder them once I am back home.
Cheers