What about a herbie overflow? Just have a straight pipe sticking up and drill a hole for a other bulkhead into the back of the tank that goes into the overflow? The straint pipe takes all your water (gate valve to adjust with precision) and you put a 90 on the new bulkhead and that's your emergency. The 90 can be high up and straight pipe can end halfway between bottom and the 90. I did this check my build thread. I drilled into the side of an external overflow but same idea.
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