I have a bit of experience designing 3D models for FDM printing, mostly ABS plastic. I would recommend designing a cover that is not a copy of the one that came with the N900.
You will need a magnet to activate the SD card Hall sensor, a white/black strip you can move back and forth over the camera cover optical sensor (turns camera and app on/off), and 'fingers' to engage the snap-in points in the main body of the N900.
thinking about it, if you have a lase cutter cut a rectangular base and consider adding printed inserts in the N900 to receive screws that run through your cover.
Of course run a high temp, I find 255C works for me, and acetone fusing after the print help make a stronger part; but FDM printing will never be even close to as strong as an injection molded part.