Floyd R Turbo
Either busy or sleeping
If you have a 5-sided internal and 5-sided external box, you put the gasket seals on either side of the glass only. So box-gasket-tank wall-gasket-box. You only care about water leaking out of the setup at the joint of the tank and outside box, and you want to seal the joint between the inside box and tank wall so that water doesn't slowly bypass the overflow box during an outage (draining your tank down). The latter is probably less likely to occur, but it's a worst-case planning thing.