We inherited an airball field from our owners outdoor field (here in Toronto when the indoor existed). The bunkers were sadly slashed by hooligans at the outdoor (no clue because the outdoor was in the middle of nowhere). Anyways. Due to the nature of air tight devices, we tried allot of different methods, from spray foam fixatives to garbage bag patches.
The best fix, that would last long and ensure a seal, involves taking vinyl fabric strips, like that used to make the bunkers (you may have to use an old bunker or sacrifice one if you don't have a supplier of industrial fabrics nearby). You clean the bunker entirely, no paint no residue nothing on it, use WD40 or goo gone, get it as clean as possible, especially around the slashes / holes. You lay the bunker flat on the ground, where the hole is able to be pressed against the ground rather. You put industrial fixitive (plastic / vinyl epoxy) to 1 side (do it in the center not around the edges). Now insert the peice entirely in the bunker, and bring the side with the epoxy to the slash / hole area with the fixitive facing out, press it down (you may get some epoxy on yours hands so wear gloves), leave it for a few hours to set (make sure too it hasn't gone over the edges otherwise 1 side of the bunker will stick to the other).
You then have 1 side of the patch finished. You then apply expoxy around the edges of the hole / rip and the patch and let that set for an hour or so.
Lastly, you make an overlapping flap of the vinyl that sit on top of the inside patch and expoxy, using the same method, you do a little overcut on the vinyl, then go to town with the epoxy and let it set.
Wait 24 hrs. Inflate and BAM!