Holes in the top of the tank big enough to get a single piece of acrylic in there to divide off the tank? If so you could just swiss cheese that piece, and slide it into a base that is big enough that they couldn't tip it. Few suction cups for support and I would think it would work well. I've had bad luck with big rays breaking egg grate and really messing themselves up when getting trapped next to, above, or below it.
Acrylic would look way better and beings it's the same as the walls of the tank I would think them to be less prone to exploration, but rather accept that it's a wall. Mine almost seem to see egg grate as an obstacle that they will stop at nothing to overcome..... Usually resulting in damage to the tank (scratches), damege to the rays (torn up when trying to weasle through a small crevis), or them winning by knocking it down and throwing the human owner into a fit of rage.
