I used use black electrical tape (2 layers for thickness) cut nice and small, took up the slack perfectly. Now I have no movement that I can tell (even if trying to force it) and you can not tell the tape is in there when the upper and lower are together.
If hinged open you can see it, but still not noticeable unless you are looking for it since it is black and I cut them the perfect size.
My rear spring assembly at the rear is tight. I have to actually squeeze the two together now because of the added pressure from the tape so be able to lock the upper to the lover without forcing/rubbing the spring assembly part.
Between that and the FAST stock wobble i was was pretty unhappy with a $2000+ gun.
Now that the upper to lower wobble is gone with tape, and I found out about the adjustment screw for the folding Hinge for the fast stock the gun feels way more solid. (I put some Teflon tape on the folding adjustment screw so it would stop loosening itself off after every couple mags).
Now to only solve the play in the length of pull adjustable part of the FAST stock. I see the new version has a set screw to take the play out, but I hear it is $400US for it...
Curious about adding my own set screw, or tighter fitting plastic white sliders that are inside the FAST stock (have to take the FAST stock apart to see the white sliders)