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