gasket placement for overflow box with back

[corpusse]

One gasket between the overflow box and the the inside of the tank and one between the flange and the inside of the overflow box seems to = no leaks.

I thought about upgrading to a nicer overflow box like the one you've pictured vs the dated design I'm using but this is just a simple fish only tank that less a few elbows and one new bulkhead I managed to plumb together with parts I had in various boxes of junk that are finally getting used.