Outside of the usual facilities, has anyone dealt with a shop that offers to measure you for a shotgun or rifle using a “try-fit” stock? I once saw a video where a custom-maker was measuring a client for a double-barrel mounted on a try-fit stock while the client actually engaged in live-fire - in-between shots, the maker would make adjustments to give the client as precise a fit as possible. This was an example of really first-class customer service.
This is typical for a shop with a 'smith that specializes in custom stock work. And it isn’t cheap.
One isn’t likely to get that level of service when purchasing an 870 off of the shelf!
If you could find someone to do that, it would be expensive. Why not get an adjustable stock?