I've been thinking a lot about adding a folding stock to my 15-22. My plan is to cut off the plastic receiver extension tube even with the back of the receiver and then bolt a piece of pic rail vertically through the plastic of the receiver itself. I'd would just drill 3 or small holes through the Picatinny section and then all the way through the receiver plastic. Then just tap the receiver holes for a 5mm screw and bolt on the pic section.
Due to the popularity of the Sig MCX there is now a variety of folding and collapsible or even fixed stock adapters that bolt directly to a piece of vertical pic rail. I've done this with two different SKS rifles so far (made them both folding stock SKSs) but not yet found the time to do it on my 12/22.

It is a simple process, but getting the holes vertical and perfectly matched with the pic rail section and the receiver does require lots of care.

Jeff