Ok, so I talked to a friend who's audio engineer and asked him about the beta91.
He said the best position to avoid the comb filter effect would be to position it directly unter the holder, so this is what I produced today:
The cool thing is: It can be produced from one piece aluminum.
I will have the form laser cut again and then I can bend it in the correct position.
This part now is made of 2 parts but that's just for my prototype.
The final aluminum parts will be a bit thicker (maybe 4mm) so it will be a bit more stable.
I also asked a drummer who uses the beta91 and asked him if he wants to adjust the height of the mic, he said "no" - that makes it easy and cheap for me!