Cutting an electrical tape shim for inside the ring will give you enough grip and friction to keep the FSP in place on the off chance you need to adjust it for windage, or ever need to replace it for any reason. That way you're not stuck only adjusting via the rear irons.
Jerry from Longshot Mfg makes rails for the 995B for mounting an RDS, and I can show you how I chopped my factory rear sight body to make room for a 30mm RDS on my 4095.