gasket placement for overflow box with back

[corpusse]

I can't remember how to do this so hopefully someone can help me out. Trapezoid overflow that has a back and is attached to the side of tank with bulkhead not silicone.

Do I just use one gasket between the inside of the tank wall and the outside of the overflow box, or do I include a second gasket between the bulkhead flange and the inside of the overflow box? Option 3 is to reverse the bulkhead and put it in backward but I have a feeling it will be tough to tighten it with the nut inside the overflow box..

thanks in advance.

[gregzz4]

My through-glass overflow box uses 1 gasket inside the tank and 1 outside the tank, and both go between the plastic and glass. Any water that may leak past the threads just travels past the gaskets into the box.
And the lock nut is inside the outer box.

Not mine, but same idea
https://i0.wp.com/synergyreef.com/wp-content/uploads/Synergy-Reef-Overflow-20-inch-Diagram-for-instructions-e1442514166620.jpg?fit=1000%2C442

[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.