I could not find a camera mount to the cap that is in portrait orientation, so made this on myself. It just slides on and stays in place with friction, no mess with glue required. 
You have to use a standard M5 nut and a M5 bolt, like on the standard action camera mount. The nut stays in place with friction, or if it is too loose you can glue it in once and that's it.

You could try printing it out of PLA, if you are planning to use indoors, but i went with PETG so that it doesnt melt on the hot sunny day. I recommend turning ironing on too, so that the surface gets nice and smooth.

Share your prints in the comments!