Scuba Steve is correct

When renting a drysuit it often becomes clear that one size does not fit everyone.
The Seal Cinch is made from comfortable, high quality, elastic webbing material, and is fully adjustable for a wide range of neck sizes.
Tired of that wet V down the front of your under garment, or worse?
The Seal Cinch holds the neck seal snug against the neck preventing gaps and leaks.  

MSRP: $16.99
Where to buy a Seal Cinch