I have a BioCube 14 and you can put it in the far back right compartment. There a multiple "slots" that the filter cartridge can go, place the cartridge close to the intakes. That will leave room for a 7" to 9" heater behind it. It may decrease the flow a tad, but IMO it is more pleasing not having the heater in the viewing area. There is limited space already so why clutter it with equipment.
A lot of other people have removed the cartridge completely and put LR rubble in the compartment. Search the threads for more info...DeMartini has a 9 page diary going on this site. She has one of the better nano's here and it is also a 14g BioCube. I too will be utilizing a LR rubble system instead on the filter cartridge and bio balls.