If you want true custom fit, and especially if it's oversized you're after, I suggest Bondo.
I've used Bondo (auto body filler) for decades to make pattern stocks for shotguns and custom stocks for shotguns and rifles.
Start with a sacrificial set of grips. Apply Bondo, wait 10 minutes and rasp. Remove Bondo and wood where you want, add more Bondo, cure a few minutes and rasp. Finish up with sandpaper. It's fun, educational and fairly cheap.
To build custom fingergrooves and palm swells it's common to slop on a huge dose of Bondo and grip the stock while wearing rubber gloves. When you're all done, spray paint and you have a functional, one of a kind set of grips. Someday, you may use it as apattern for a gripmaker to duplicate in fancy wood.
I carved these without going the pattern route, but they illustrate purpose-made, one-of-a-kind grips. In this case, for DA speed shooting where conventional grips tend to slide down in my palm before the cylinder is empty. The "beavertail" spur at the top prevents that.
