I'd rethink that. You should consider adding a layer of glass or kevlar around the end of the stock to cover the core. ACP or Soller Composistes might sell Kevlar in small enough quanities. I'd wrap it around the end and have an overlap of about .75" up the sides. We glued on our recoil pads with Goop, so you have to have an inpermiable layer there to keep the solvent in Goop from attacking the foam core.
Use good quality epoxy carefully measured. I used to weigh epoxy for small jobs but now I use separate syringes and mix to the suggested volumes- in the case of the stuff I'm using now it's 4 resin/1 hardener. it's Jeffco/Tap epoxy, which isn't the best but it's cheap.
When we did some "pounders" at the shop I worked at we tried to add as little bondo on the seams as possible, as bondo packs on the weight quickly. To finish them we'd spray with "Rocker Panel" spay and paint black.