The CFS500 and aquatop 500 both appear to be too big to fit through the cabinet based on the specs i am finding online. That leaves me with a few good suggestions: Eheim 2217, Fluval 405 and the Magnum 350.
For the Magnum, I do like the idea of it being clear and seeing how close you are to needing to clean it. It is also the cheapest of this group..about half the price of the Eheim on some sites. Is the Eheim twice as good to warrant the extra cost?
Also, this is to fill a secondary mechanical filtration need (water polishing,) not bio as the sump is handling the nitrates fine.