A very small amount of PVC glue with a liberal amount of pvc shavings added to it. Allow a fair amount of time for the shavings to "melt" in the glue, leaving a thick "jelly like" consistency. If it is not thick enough, add more shavings.
Coat the screws with ARO, or better yet, melted candle wax to act as a release agent.
When you have your "putty" made, fill the hole in with it , and pack it tight. Insert your screw, and snug it into position the best you can. Leave it at least 12 hours and then snug the screw as you please.
This method also works very well with any 2 part epoxy such as JB weld, as long as the part you are putting the epoxy on is roughed up.
My dad once did it on stripped VW bug sparkplug holes, and it lasted 75,000 miles. (JB Weld method) They were even able to tourqe the plugs to factory specs.
Just my .01
-Moto