Yeah that's what I'm talking about. Rtv works well in conjunction with some duct tape keeping it in place as the rtv cures. Even then it's no absolute promise it won't leak. I could see using one of those little tubes of kitchen silicone to lay a good bead to keep the epoxy in.
I've gone and done the small metal working tool and a small butane torch deal with the 5 minute stuff to manually shape the epoxy. The real key is to warm up the spot a bit before putting the epoxy in, which is best if stirred really vigorously for about a minute or 2, this way the stuff will set pretty quick right after putting it on the spot. Mark with a sharpie so you know where to keep the wall. If you wait till the right second the weld gets that play dough like consistency and you can trim it quickly with an exacto. Waiting until afterwards just use a sanding drum on a dremel for touch up..
What's up with your epoxy? It looks like Elmer's glue or semenal fluid or something. I've played around with the white putty you knead, the jb waterweld, it does have some interesting strength to it I've noticed, it could probably be used effectively since it's just a simple barrier. Would be easier to apply too