I designed the original M-Mate that Luigi sells, it's a replacement baseplate for the M8/M9 that has battery and SD card access. CNC from aluminum. We were reasonably confident that the fit was correct, but even just a smidge of a hair of a millimeter can make the difference between a perfect fit and a loose fit. We took the camera to the machine shop and they built three or four units, making minor adjustments, which we fit onto the M8 until it was perfect. Machine time and machinists are expensive, so you need to be very very close before doing this.
This project certainly is doable in aluminum, but to make it financially viable you'd need to be able to sell the productions, and to do that, you'd have to offer some additional functionality. Not sure what that might be. 🙂
There are some desktop CNC machines coming out, for prototyping and low volume. I'm not sure what kind of tolerances these machines are capable of, however. And there aren't any 5 axis that I know of!
Protolabs in MN can send you an aluminum part in three days if you don't want to mess with all that, and they do an absolutely amazing job. But you're talking at least $1,000 for the bottom plate, and probably a lot more than that for the top, just for the prototypes. If you're on a budget you'd need to make sure your measurements are absolutely spot-on. For the designs that I currently do, it's pretty common to have to send three sets of CAD to Protolabs before it's perfect.
Before doing that, get a 3D print made. That will reveal many of the measurement errors, but not all.