They do make air driven CC skimmers that go inside the tank. The Lee and the Berlin CC work like that. They are not very attractive though. I am using the small Lee in the propagation tanks right now and, believe it or not, it's doing a pretty good job. I don't know about the Berlin, but the Lee doesn't have a very good system to anchor it inside the tank. Mine is bad about coming loose and floating around. It is kind of a PITA to clean and you have to drag it out and change the airstone once in a while. I have a small skimmer from Aqua Clear Aquatics in Jacksonville, Fla that would probably work in the tank. You would have to leave about 6" of it sticking out of the water. It uses a Rio 600 (yuk) and also works fairly well. Not much help I know, but it's all I know of.
, you'd be better off to try to find a way to use a Prizm or Seaclone on the back of the tank.