I think that if I do a 60-cm kammback (horizontal length between hatch separation and rear bumper) and a 12° angle, I should only have the upper 12 cm of the rear glass blocked (12° is approx. 20%).
I have seen that the original covers for one-mirror AXs can still be purchased.
I can even get the left one that was on right side drive versions. It will look stock, they are just black.