I found a lot of the camera mounts far too complicated for my needs. I personally use a Polaroid Cube+ on my x210, which has similar dimensions to the Runcam 3 and GoPro Session cameras.
This mount will need either a battery strap or strip of velcro to secure it to the top plate of the x210 (see images).
The aperture and side walls are dead centre on Version 1. On Version 2 they are slightly raised, as I found the mount sat slightly lower when the battery strap is tight.
I would advice you print face down, as it not will need supports. The strap should attach to the bottom first, then go under the top plate of the frame, up through the slot behind the 'ramp' over the camera and through the top slit.
Check out my other Things for a VTX tray for the x210.
Should print fine without supports, as the vents are pretty small.