Aaah, sorry for the confusion! It made sense in my head, but, without seeing the tank I understand why "omg there's a gap" could be hard to understand. So, I took pictures.
So, this is my tank! That big plastic tub is their water dish (currently empty, and will have some fake plants and the like going into it, as well as a slope to make exiting easier). And, maybe, you can see the gap I'm talking about, between the water dish and the egg crate wall holding the substrate back.
Here are the close-ups of it. The cork doesn't lie flat on top of it, unfortunately, so the toads could get under there and get stuck in that crevice. I've been thinking about adding, like, a bit of egg crate in there, or maybe some type of foam filter. Anything put in there will tumble out when I remove the water tub, so, it can't be more substrate, or gravel. Larger rocks might work, because I'd at least be able to put the rocks back when the tub goes back in.