Seems to have stuck! I scraped/peeled away as much of the old adhesive as I could first

you could use some artificial sinue (leather stitching kind is ultra strong) and put an extra layer of reenforcement on that

Thanks for the recommendation, I hadn't heard of artificial sinew.

I think this spot was glued and not stitched for water resistance. The top fabric was coated, too. So, I'll see if the glue holds without stitches, it shouldn't get a lot of mechanical stress.

the nice thing about the sinew is that it is waterproof and most of it will come with a wax coating that will help create more of a seal

Yeah, and you can also always put glue on it after you put it in there, making it seal even better!