What is taking up so much space under the tank that you have no room? The area you suggested is too small for most skimmers large enough for a 50 gallon tank.
You have to spend a lot more money on an HOB skimmer to get a decent one, than you do an in-sump. Remoras IMO are a waste of money, I've had two (the regular and the pro), and I think the only people who say they are any good are people who have never had a better skimmer to compare it to!!
I think the only HOB skimmers worth buying are Deltecs at minimum. You can spend half the money of a Deltec and get a comparable Vertex in-sump. I would suggest you find a way to make room for a 20 gallon tall tank for a sump at least.