Have you looked at the Two Little Fishies Phosban 150? Probably a little bigger than you were thinking, but hangs on the back easily and a decent price.
If its for GFO or Biopellets you should look into a pax bellum ARID system. If you check their vendor page you can enter your tank and maybe get chosen to test one.
I actually saw one last week and it is a pretty cool device. The person who had it really liked it. Once I get my new system up and running I might get one.
It depends on what your using it for. I picked up two MR1 reactors used for like $40 a pop. The shorty one they make would be a nice option for you. The cleaning on them is so easy. What ever you do, Stay away from the Bulk Reef Supply reactors.