Ahh, a varmint entry tactical....
hroughWe sell 2" ($15) and 4" ($25) Picatinny rails with appropriate holes and (inner) back-up alu plates with threaded holes, but they are "flat" dimensions in the 1/2 x 2" contact area. Your FF HG is obviously a curved surface.
There'd be too much wobble potential and possibility of tearing loose for drilling and tapping the tiny diameter screws that would be appropriately sized.
I would through bolt, using washer and NyLock nut. I would take up the 'slack' from the curvature issue with thin neoprene, maybe even self adhesive strips laid along the two edges. Any way you do it, you're going to need to fill that gap to avoid loosening and wobble/stress. A machinist could make you the appropriate filler, I have made many a botched attempt at doing such things for my personal use, that's why I am not a machinist.
hatand it is designed to fit the curve. $65
Doc @ M&A Parts 847 550-8246 9-5 cst m-fri