Holder perfectly fit to DJI RC. You can instal it directly to the bike bridge or put it first on the RC. In my case bike bridge is 38mm.
To join all parts you should us inserts M3x4mm and hex bolts M3x8mm.
The holes in the base are arranged in a square so that you can mount the handle vertically or horizontally as you like.
I uploaded source Fusion360 file so you can change only bridge holder to your size.