Seems rather extreme, to me, to cut away perfectly good fore-end wood and patch in a new block of wood, for a minor problem. My suggestion will no doubt attract incoming fire, but here goes: if it were my rifle, I'd superglue a layer or two (or three) of hardwood veneer, cut to shape, into the trigger guard recess. I'd get the correct shape by fitting a piece of paper first, to make a template. If the front needs to be filled more than the back, I'd build up the front a bit more , and sand the layers flat. When cutting the veneers, I'd run the grain diagonally and criss-cross successive layers. When the superglue is set, I'd trim out the magazine slot and screw holes with a hobby knife and file. Nothing will be visible when the trigger guard is installed.
An even simpler solution, which is reversible, would be to lay in inlays of smooth card (e.g. playing cards) into the recess. I've used playing cards to make shims before in other situations, and they worked just fine. Cards are completely incompressible. In fact, you could start with cards and once you are pleased with the fitting of the trigger guard, you could replace with veneers...