They are pretty clever, they won't go in holes they can't fit in or out of. I have already had to enlarge the hole though in 3 weeks since I put it in and he fitted easily before. The end is totally open and 5 inches so they will never get that round and even if it did it would not be a sudden surprise.
The smaller ones at the end of the tank were for the tigrinus and these are gone now, the eel used to put his head in there but would go no further.