Have the action settled into the wood. I had the stock for my DWM set on the bench as a visual guide and for measurement checks.

I made a start on the jig to allow the use of the router to do the blind mag. I cut a block to fit into the recoil lug and fixed a slow wedge shaped piece to it so the top of the guide was parallel to the bottom of the stock.
Careful measuring of the internal mag dimensions and allowing for the guide collar on the router base required a 1/2 inch over the required size so I should get the correct finished size. I will try it tomorrow on a scrap piece first then one the practice stock. Going to be an interesting trial.