Well, there may be a solution here...
The Springfield 1903A3 and earlier actions have magazine cutoffs on them that allowed them to be used as single shots.
Maybe get yourself one of those actions and simply use it with the cutoff turned "on"?
If that's not to your liking, I don't think it would be very difficult for a competent gunsmith to take a Mauser, or any other, action and remove the magazine and simply put in a blind bottom.
Remington's bolt action .22 line from the 1940s to the 1980s used identical receivers with an open bottom.
The single shots, however, had a blind bottom pinned in place. If removed, adapters could be fitted to the same receiver that would allow feeding from a box magazine or from a tubular magazine.
Very simple, very elegant.