i used a prizm skimmer on the back side of my 12g nano and it worked great. you just have to cut the lid for it to fit right but there is no pump inside the chambers in the back that bulk up the area.
just an idea.
i wouldnt say it performed as well as any other skimmers but it did work. i emptied the cup about 1 time every 2 weeks but i did weekly water changes.