Marc what you're looking for is what's called an 'S' section rubber,as opposed to the 'H' section commonly used.
I'm sure there's a supplier in the U.S. but over here I use a company called
C.O.H. Baines they have an online catalogue showing all their profiles in 1:1 scale.
Might be worth a look even to see if they have anything close.
Ideally you'll need someting with 1/4" glass groove and 1/8" pinch weld groove.
I hope this in some way helps.
Good luck.
