Siliconing overflow to established tank?

[Lsufan]

A lot of the 4 sided boxes come with the gaskets & bulkhead, so u may want to check & see. The only issue with those are the hole in the overflow is usually predrilled so it may not line up with your existing hole. In your case u would be better off with a box that doesn't have a predrilled hole so u can drill the hole yourself so it lines up with the hole in the tank.