I was thinking of drilling a hole in the return as well. Figured I might as well have multiple safety mechanisms in case one of them fails. The check valve is best because it allows no back siphon, but then in case that fails a siphon break hole would only let a couple gallons of water go down into the sump before the hole was exposed and the siphon breaks. The return is going to be a single 3/4" lockline with a flared nozzle. I positioned the bulkhead as close to the top of the tank as possible because I want the return to be close to the surface for surface agitation.