I found two options for quick fairly rugged repairs:
1- For small tears Gorilla glue
applied to a cut piece of rubber (innertube, gasket blank). Clean the tear, smear GG onto the underside of the top and the rubber, hold down from above the top and from under on the rubber for five minutes or so, done. Watch out for excess on your hands, or you'll be part of the patch. I've used GG for wetsuit repair and it was good up to four inch slices, still working. Top repair is 2 months in, still dry.
2- For plastic inserts above doors, at tub rails, and for all canvas seams the solution so far is from Michaels
. I asked for heavy duty needles (explained the issue) and the sales person recommended sinew
. I pulled enough sinew through large needle (about 30") to sew with. Then, knotted the end, and pushed the needle through first section of plastic (carefully, best to find an existing hole otherwise they break), and went around the outside edge and repeat equally spaced threading. At end loop and pull a knot tight to tie it off. One frayed and I applied a drop of GG to lock it in.
Hope this helps...