I have also been after an RMR adapter like this for a while but no luck trying to source one here.
I have one of these: https://www.davescustomairsoft.co.uk/rmr-red-dot-sight-adjustable-brightness-black-de?search=Rmr adjustable
It does fit onto the FNX with the picatinny adapter it comes with but it doesn't seem very stable and it sits very high, totally obscuring the iron sights.
I've asked Fire Support to try and order one in from Ace One Arms directly, I'll let you know if they get back to me.