Whitey, one of the first things he'd want to make would be a properly-sized tapered reamer for the circular door seat. I believe it would be simpler by far for him to do a circular door rather than the elongated one, and a suitably-larger stock recess that 'turns the corner' would allow a place for the Lyman 48 slide, as already mentioned.
The main problem with this approach is the acquisition of enough milsurp circular doors, the milled buttplates are now bringing astronomical prices. He'd probably end up having to make his own doors.
Considering that a newly-made trap-door buttplate costs at least $150-200 and frequently more and almost certainly wouldn't fit the original stock anyway, I'll bet that he could charge a very pretty penny for a nice conversion of any existing M70 plate.
I can send you a milled milsurp buttplate with circular door for examination/conversion/destruction if you wish, just say the word.
Regards, Joe
