To be honest, if I was going to do it, I'd use real parts...cast off. Still, the way I understand, they have to be regulated, or calibrated...however you want to say it. When they were done originally they were done and serialized for a reason. That's why some just use the screwless mounts to make a #4 shooter and don't try to make it look like a proper #4T. Like a "B square" for lack of a better example. There IS a UKicon site that produces what I think are real parts, though they're aftermarket for sure. We've discussed them here before...

Mount... Fultons of Bisley - SCOPE MOUNTING BRACKET FOR SCHMIDT AND BENDER L13A1 SR1

I know we had the blocks somewhere too...made in the UK I think.