I'll add to what Pitlamp says about Aquasure; it's intended as a sealant (just a layer of it on the surface) but it's a very effective glue provided you hold it in contact. It also help to leave the coated surface exposed to air for 15 minutes before you put them together, as it cures using moisture in the air, and leaving it exposed allow the cure to kick off before you cover it up. It will stick to anything, even the cap of the tube, so when you finish, clean the nozzle and cap put the cap on them put the tube in freezer (not the fridge) until you need it again.
The other thing that's occurred to me is that if there's a canvas layer in the welly behind the split, working some Aquasure into the split could work well, lift the edge, poke it in with cocktail stick, wedge it open with the stick for 15 min, close it up and apply a layer on the outside. The inner layer and the outer will be connected through the split and that will resist pulling off.