On Marty's Funhouse thread I posted a description of how I flattened and re-glued my inserts. This is what I said and a picture to try and explain with greater clarity.
What I said was ....
I have recently done this on my Gorgar and first used pretty fine wet and dry to level the inserts as they were quite bowed - one could easily see light through the gap when help against a flat surface. This sanding caused the inserts to loose a bit of height so they then sunk below the PF top surface level. To overcome this I put a tiny splob of silastic on a matchstick and then placed the matchstick on the insert ensuring the matchstick was really flat, close and aligned to the top surface of the insert. Use two matchstick for the bigger inserts and set aside to dry, insert on top of the matchstick. When you go to glue the insert in the overhang of the matchsticks provides a support on the top of the PF. When the insert to PF glue is dry you peel the matchstick off and are presented with an insert true and flat to the PF surface. Sorry it's a poor description, but not that hard. I will post a pic on my Gorgar resto thread.
The glue I used was superglue gel - was in hardware stores. Pretty cheap and a green cap from memory. I used it sparingly and ended up with a bit of a gap around the insert, it's enough to fill with wax when polishing. I read somewhere that this can be filled with normal superglue, but I haven't tried this yet.