The replica clamp I have has a plastic seal, I believe the real MPP was also plastic not rubber, though I could be wrong.
It would be difficult to find a replacement for just the sheath without buying a whole new clamp, but you can probably make one with some black styrene sheet and cut some very thin strips to glue either end for the ridges.
I’ve also heard of people using old 35mm film cases, but I think they were a little too thick.