The mount design depends on the reduction setup you use, because you want the prop flange to be properly located for thrust angle, height and center. I had to determine this by looking at the blueprints and a measured drawing o f the Rotax 912 because Zenith could not provide that information. Once I figured that out I built a firewall mockup from angle iron, and made a mock up of the engine mounting points with the engine the proper place.  Then with the two mock ups I fabricated a mount from mostly 3/4' 4130 same wall as specified int he prints.  Took a bunch o f time and one messed up mount, but was interesting.   This spring I did taxi testing and except for some excess vibration at 3k it seems to work good.  Then it got hot, then i got an inner ear problem, and I am waiting on my head and t e weather to get back together for flight testing.  I plan to experiment with rotating the prop on the flange to see if it affects the vibration.  Taxi testing was a thrill as the plane seemed to want to jump off the ground.