It wouldn't be that hard, just laborious. It would involve milling the oval hole and filing the hinge cutout and then cutting the door out of a sacrificial butt plate with the same striation pattern. The real work comes in when engineering and fabbing the hinge and spring system.
But the real problem would be milling the lip into which the door lands when closed, due to the curvature of the surface. I suppose a better machinist than me (LBK?) could see a simple solution to that problem. (It is what deterred me from making a hinged butt trap in a Niedner-style plate after a buddy gifted me a beautiful cast lid for the project.)
Looking at the less than perfect pic of this butt trap, the workmanship of the hinge system doesn't look like Savage factory quality (especially since it wasn't a production item and would have been done in the tool/experimental shop), but the lid/opening itself looks very well executed.
