The main thing I am worried about is that it won't be planar. You can take a rectangular frame, and twist it, just by rotating the joints. So quite a tiny error in the joints could produce a few mm error in the frame. This will mean that what you end up milling will not be flat either. I have a particular problem as the table I have just built for it is very definitely rough woodwork, so I definitely can't rely on that for a plane. Horizontal is just a convenient plane to use.