As promised in Backcover/Body Replacement thread, I'll release all project files for produced components under some Free license (might need to evaluate best one to use, Creative Commons comes out from top of my head). For a backcover, it is actually 100% ready - just checking for elements that may prove problematic to anodize and, eventually, correcting them.

If you don't care about such details as anodizing-friendly shaped, I may send you current, pre-polished version of files. *Although*, I don't have STL (or other 3D) files for backcover, using 2D CAD drawings from various planes, and converting them to g-codes for CNC milling/3-D printing (inc. enormous amount of manual work, i.e. manual g-codes correcting/writing). Of course, g-codes are also things that I'll be releasing, and may share a non-polished (yet) version with you.
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Now, if for some reasons you absolutely need 3D files, I would recommend OpenSCAD and it's (semi) automatic 2D->3D extrusion from 2D CAD files. I haven't done it myself, but OpenSCAD website have some tutorials on the matter.

Having STL files greatly aid in automatic creation of g-codes (99,99% automatically for 3D printers - importing STL files integrated in most FOSS solutions you only need to adjust some details in rare cases. For CNC milling, it's only semi-automatic, i.e. might require re-working some stupidities in most cases, and requires external tools, like awesome PyCAM). I would certainly be able to finish repl. project earlier, if I had access to scanned 3D models of N900's body - but, sadly, attempts at providing those by supporters failed up-to-date (thanks for trying, anyway!), so I had to fallback into doing manual work myself. 3D scanning isn't yet that available :/

Cheers,
/Estel