Thought you'd like that one. The stash I actually had a friend help out on. I'm not a skilled carpenter, nor do I have the tools needed. That said, it is very doable for the Mini, just some fairly precise inletting and sanding to fit.
The addition of the spacer worked because it was a compact stock, extending LOP, and I didn't have to risk mangling the stock. For a standard Mini 14 stock (if it's wide enough), I would fill the "shoulder curve", making the butt flat and work from there. You can find used butt plates for pretty cheap, try picking one up and see if it'll fit.