Pretty much any deck fitting should have a substantial backer plate. I like G10 for this function, although fibreglass sheet can work for less money when less stiffness is required. I keep some of each in different thicknesses on hand.
But how do we hold the backer plate in place while we drill the holes?The best way I have found is to epoxy it to the underside of the deck. So doing also stiffens everything up because the thickened epoxy fills any voids or curves.
Cool, but how do we hold the plate in place while the epoxy dries?
Simple, a dab from a hot glue gun. In the photo above I have applied the epoxy, but left a small area for the hot glue. Hold in place for 15 seconds and it stays put. Works a treat.
Once the epoxy hardens up, drill the hole(s).