i "macgyvered" my own overflows out of biowheel housings... cut a hole in the bottom, run a pipe in, goober with silicone, VOILA! self-starting overflow. i cut off the intake tube at the desired water level, so it will only drain tank to that point, and setup a durso standpipe in the box above the impeller level, so the box wont drain below that level.
i also have a shutoff in the line to the sump so that i dont have to turn them off when i want to empty sump. i just close the shutoff, empty the sump, fill it again, then open the shutoff. i dont even have to shutoff any of the pumps, since the return pump is in its own compartment and is setup to remain submerged even if it pumps out as much as it can.