im running several tanks with modified bean animals, i never had the room for the emergency drain, My main tank i built a glass external box with 2 3/4 pipes, one full siphon one partial, in the tank i had 2 holes drilled , with a piece of glass on an angle setting the level in the tank.
in my farm tanks downstairs all 40g breeders i drilled 2 3/4 in holes right at the bottom of the rim, i should have gone down a bit lower, inside i built an acrylic box it was about 6in high 6 wide and 3 deep, this was silicon around the bulkheads, about 1 inch below the top of the tank so far it all works great, apon startup the top tanks get right up to over flowing before the full siphon kicks in, this would be solved by drilling the bulkheads down farther from the rim