[JustOneMore20]

If you buy the versa top or Perfecto glass tops, there is enough space in the back for a HOB filter and the heater cord and such. You shouldn't have to cut it. The All-Glass versa top one comes with a plastic strip that you can cut for the heater. I don't use it, but you may want to because without it there is about a 1-1.5 inch gap (and the eel may escape!).

I'm not sure about the Perfecto top...I will be getting that brand for my new 55g...but haven't got one yet. I like them better than the top/hood that comes with the tanks. It looks better IMO.