I have a wooden puzzle that is very old. It is about 3/16" thick. I built up the sides with foamboard and layed a piece of mylar over it, pulled as tight as possible and atg it down very close to the outer edge of the puzzle. Then put a matboard on top of the mylar and conservation glass. The problem is when I turn it over to close it up the pieces shift some and some of the pieces stick out.The larger pieces on the top rest on smaller pieces so it seems to let them shift. Also one of the larger pieces on the bottom are warped so that sticks out too. The customer doesn't want it glued down to save it's orginal state. Any ideas on how to get the pieces to stay in place?