Our returns will be above the waterline, and we'll be using a secondary container in the event of back flow. Our sump will sit inside the aquarium we cracked while drilling (we're patching it up), but the sump itself will accommodate the backflow (providing I can do this and keep the pump submerged).
Once I figured out exactly what to google, I found a Canadian source for the check valve. I think they're a board sponsor, but I need to confirm that before I post the name of the outfit.
The check valve is just a little bit of insurance for us: in case we mess up (which we will at some point
).
Edit: Yes, they are a board sponsor. Go Reef Inc. has a great selection of check valves. Ordering was easy, and shipping was $9 for the valve and three bulkheads I ordered.