That is pretty tight. From the pic, this looks like the inside of the tank looking out..... I would flat-spot the top of the piece on the outside to just fit under the frame, liberally apply some aquarium silicon to the gaskets, and fill the gap between the tank & bulkhead with silicon as well. Tighten firmly, but be careful not to over-tighten (it is plastic). If you used enough, you should have some squish out.... use this to run a bead around the bulkhead with your finger.