I have the same reactor. I wouldn't want to impede the flow in any way.
With gravel, I would put it as close to the substrate as possible, but leave about ½" clearance. With sand, however, it's gotta go higher.
Mounting diagonal might work, but I wouldn't like the way it looked, plus I've always wondered if internal powerheads like the Rio 10 will wear out faster if you don't run them upright.