Sad news, the glass between the two overflow holes cracked while I was reinstalling the bulkheads. Feeling like a dummy...
While not something I'd do in a show tank in a living room, I would consider silicone'ing a piece of supportive acrylic to the inside and using it still if it's a garage frag tank. Really lather on the silicone to the back of a large, but thin, piece, trying to get some on the crack too. Let it dry, see if it holds water, maybe put a big bead of silicone on the outside too. Alternatively do the same but with epoxy.
Might be an awful idea, but lowboys are so shallow there's not really that much water pressure on it. Would also depend on the crack.