I found many 90 degrees adapters, but none worked for me. So I built my own one.
Strong built so it works with smartphonegrip aswell.
2 x m3 ruthex inserts needed for this and 3x m3x6 bolts to mount it to the existing holder.
If you're looking for a dual holder with 90 degrees, here you got it: https://makerworld.com/en/models/493674