RDS-C6

The main reason Red Disc Seal works is because how the o-ring is nested inside the disc. Disc compresses the o-ring so it seals around the pipe. The disc needs to be thick enough to house/compress the o-ring.

That said, yes, Red Disc Seal can be thinner. One option is RDS-C6, only 3mm thick. C6's design pushes the o-ring nest inside the wall. Therefore, making it thinner than RDS-C8.