This was of course something i considered when i started to look into making my own battery, but when i discovered that i could by the mugen cover i abandoned it.
I think it will look bad... and the phone will be more chunky with a square hanging out in the back, mugen made the cover pretty good, not excellent because then there would be no rattling sound from the camera shutter switch.
I still have an extra normal cover laying around.

But again the mugen cover works for me, and these days i am really busy with a lot of things so i don't use my N900 much, so it goes for 3 days before recharge and this is still with all functions on (BT, WIFI etc....)