Any search for waveguide bends yields such large numbers of suppliers that it is surprising that references to papers, abstracts, handbooks, etc. with design information for these is so hard to find. These have been around so long that much of the recipe has to be prior art. Searches in this forum do not yield much.
For 90° mitred E-Plane bends, it seems that having a 45° reflector surface chamfer cut enough out of the corner profile that it ends up a quarter-wavelength long is a traditional method, but I have seen multi-facet flats in a tight bend, so there must be more ways to do it. H-plane mitres are seen on hybrids, but I find no design rules.
Curved bends only work well if the radius is generous enough, though the values seem standard and traditional, maybe found by tryout.
Other than cut-and-try, or maybe just exhaustively simulating them all in full optimize mode on every variable, do we have any pointers to some even basic published design rules, for those occasions when there is just not enough space to be bolting together bends with flanges?