You could order a stock from Richards Micro-fit and have them not machine out the mag well. Down side is you have to wait 2 to 3 months, and do your own finishing.

I don't think I'd fill in a mag well as you mention. That big of mass of material will get hot and expand as it cures, particularly if over catalyzed. Not good for the wood. Also good to know is that catalyst is not used up in the cure. It is free to migrate afterwards and will migrate into wood, and onto barrel. In small amounts it's not so noticeable, but a large mass probably will be.

That's why I prefer to use epoxy when ever possible.