Any time you cut in a board, it's prudent to laminate over affected area with fiberglass or carbon cloth. Epoxy, even when it includes some chopped up fiberglass cloth, just doesn't offer sufficient strength. The lamination step integrates everything into the structure of the board, and it serves to provide the necessary reinforcement outside the boundary of the inserts, especially if the area will see stress of any kind. Given the fact that the weight of the board will balance and leverage off any inserts you install, quite a bit of concentrated stress will be present.
Also, if I were you, I would go for larger, more robust inserts. I would recommend checking out the molded inserts that Chinook makes for footstraps (view at Fiberglass Supply's website). It's important to establish a larger bearing surface that will better integrate with the surface structure of the board (through the use of the fiberglass/carbon cloth), as the internal foam core doesn't offer much support to speak of.