I doubt you will find one, have you tried contacting coralife? Maybe a small bead silicone and let it cure to act as a gasket before reinstalling lens.
Sometimes local fish stores (LFS) have used aquariums that they've taken as trade-ins and practically give them away to new owners just to help get them started. Perhaps you can find a used aquarium like yours at a LFS and perhaps they'd let you scavenge what you need. Just a thought. Good luck.