I think that the manufacturer is letting you know that any screws that are used to secure the tank (through a flange perhaps?) must smaller than the hole drilled in the tank. That would be to prevent what I would call point loading where the screw acts like a wedge and splits the tank.
That is my best guess....
Bruce
I will rebuild the box the same way you made it out of solid wood to prevent swelling and yes it supports very well. I am concerned about the tank not being real tight against the pan.
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