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.