Quoted:
I had to grind down both sides.  Do a little grinding at a time and test for fiit.  I used a dremel. Once you get it to fit you'll probably notice a little wobble.  To some here, I've read it was no big deal.  It's just a slight wobble.  Notice the inside rails on the stock GSG buttstock is rectangular and the POF is tapered?  That's your wobble.  I used JB Weld to layer the upper tapered part of the POF rail.  I  layered it up a little at a time letting the JB Weld dry and used the dremel to grind down the rough/ uneven spots on the layer.  Test for fit, and apply another layer.  I did this three times until I got a good perpendicular angle with a flat surface and a rock solid fit with no wobble.  It was really easy, even for me.  The POF is a huge improvement over the GSG buttstock.