I'm thinking about doing the same to my Dark Menace, so same question.
Also curious about type of material.
The 0 rings inner diameter are approx. 3 cm or 1 1/4 ". The brand I like the most is Master Plumber Delta 0 ring kit. Package # 649 with 4 rings ( x2 thin, x2 thick ). Should be slightly difficult to place and not roll around when seated in the grooves and elsewhere.
