I'd go with a 20g, but a 10g would suffice. The glass on the tank is tempered so you can't drill the tank and I don't know if you could find an overflow box that would fit very well. If you took measurements you might be able to find one.
What I would do is get yourself a 20g and drill it. Put that up above your tank, put a pump in the back of your red sea and pump the water to the 20g and let gravity feed the water back into the red sea. In essence, the red sea max would act as your sump with all your livestock in it.