I am not sure what the pros use but I bet it's close to the formulation
for a product sold at hardware stores called "Goo", specifically "Shoe
GOO" (one of the incarnations of "Goo" brand) I have used in boat
building for bonding material and foam, etc. I have used in conjunction
with small fiberglass patches for patching everything from shoes, to our
white water raft, on the fly patches that are flexible and lasted ten
years or more... Grab a tube for ten bucks and a scrap of canvas and
give it a try... You will find that a thin layer on each surface, wait
five minutes, wet them with a bit more and press together not pushing
all of the glue out of the joint will make a great bond that is as
flexible and durable as the canvas itself, maybe more so... When dry,
you would think it was a piece of rubber...
Either way, you can try a tube for less than ten bucks, look for it, let
me know how it went...
Good luck..