We offer this Conical Inner Neck Seal design with an 20cm (8-inch) mounting diameter that is found on Viking brand and many other brand drysuits. If you are going to glue the seal to the inner part of the suit the inner seal design may work better, although some people still prefer inner mounting a standard bellows design neck seal. Conical inner neck seals are available only in 0.7-1.0mm standard thickness and two sizes; the medium is 25cm (10-inch) in circumference and fits 28-35cm (11-inch to 13-inch) necks, the large is 30cm (12-inch) circumference and fits 33-40cm (13-inch to 16-inch) necks.
Sizing video:
https://youtu.be/2rGTvinn19E