I've got a herbie now, which is all you can do with the two holes in the Marineland overflows. I've got two of them, actually. I plan to add a third drain for the emergency, which (with a little extra plumbing) basically makes it into a bean animal. A 1" bulkhead with an elbow open just above the normal water level should do the trick. It doesn't really need to be inside the overflow to do its job. I'll tear out one of the overflows and cover the holes in the bottom, eliminating one of the herbies, leaving three holes for the bean animal setup.
It's either that, or sell the tank, buy an undrilled, and do the whole bean animal setup from scratch. Mods to the existing tank would be slightly logistically easier for me if they are practical.