Have seen that there is quite some interest in how I made myself the mounting brackets for the shaft drive construction in my hardtail frame, here are some more details.
The basics are pretty simple. Get yourselves some thick mild steel plate. Mine are 12mm thick. With some cardboard you make yourselves some templates, copying the holes from the axle and the mounting bolds of the shaft drive bell. Since you must be able to move the drive shaft a bit when mounting the rear wheel into the frame, I made the 2 plates detachable by drilling and tapping some mounting holes into the side plate.
If you keep these 2 plates nice and square you will have a strong supporting structure to be added to your hardtail frame.
The reason for making the sideplate (welded to the frame) as long as I did is that this allowed me to weld alongside the pipes (frame tube notched around the plate). This way the twisting forces of the shaft drive bell are supported by the full frame triangle which is about 65cm long in my frame. When you make the length of this triangle very short (by welding the 90 degree support bracket directly to the frame), the vibrations will eventually break your frame.
Hope this helps a bit and enjoy building your own.