I appreciate the suggestions but I'm not a fan of the look of those. I'd prefer black wood, I'll keep looking.The natural wood one I posted earlier is this one from target. They also have a black metal version I really like and I've used for the double matted Horizons print of the same size from this year's Epcot festival of the arts. The black one is a little easier to work with, as the clips on the back are completely adjustable (bendable) to fit a thicker double matte, though they fit the Horizons print with no struggle. With the wooden frame I used for Living Seas it did take a little pressure to force the back on, but it works fine with a little pressure. Honestly, if you want a simple black frame, that frame from Target is really nice.
Edit: These ones say double matted. I'm gonna try them: