I bought a neck seal from DRIS some time ago to fit in a Whites Fusion. I also found it needed to be stretched. The Whites had a very small (~5mm) overlap between the seal and suit and was stitched in. For gluing I wanted a larger (~20mm) overlap. DRIS also gave me good advice, and said stretching was required, and what they also do. I used their heat glue tape. I had to make sure the suit was taught so that the neck was held open, then glue the opposite sides of the seal so as to progressively stretch the seam and not have the suit 'bunch-up' as the seal tried to pull-in, also needed to allow the glue to set otherwise the seal 'slid' in. It was a slow process but very successful.
Thanks DRIS.