Apologies have been away h past few days so I missed this
Are you trying to mount an actual RMR or a dr style sight?
If your trying to use an RMR then its quite easy, I started with the Glock mount that came with my sight and ground off the rectangle part that mounts it to the Glock, I then cut the plate down to the sight was just at the front of the sight cut out on the fnx already. Lastly drilled two holes to line up with the threads already in the bbu and countersunk them and that's it all done. id say all in took about half an hours work and looks awesome.
Excuse the self vector promotion its the only pic I have on the pc of the fnx