Your biggest problem with be the inletting of ALL...of the different rifles available, that you want to do.
Many years ago, I met a guy that was making custom bullpup stocks for Ruger .10/22's (mine looks like a Steyr AUG with a Mannlicher forend). That's ALL he did. After a few years and people bugging him to do this rifle and that rifle, he started to branching out. BUT, having guns on hand to cut out the center, you must have the gun in hand. Most people aren't too receptive to shipping their guns across the country, so finding someone close by for xxx guns is good.
Or, yeah, sending a private party gun to you is a perfect way to have the perfect fit for the customer. I was lucky, he was already doing the 10/22. He was also willing to manufacture a different scope mount than he had on hand. I drew him a design, he machined out of aluminum, perfect. You'll need a library of inletting dimensions of the given rifles that you want to build stocks for.
Good luck, have fun.
Mike