Check the celotex. It may be where your warp is. If it is and the particle board is flat just screw it to the particle board. If te particle board or the door or both are warped try this. Get a piece of 3/4 MDF and rip it in half long ways. Flop the pieces together and cross screw them together. Screw from the top and bottom. It will be straight and stay that way with the screws. Then put your celotex on the top of that. I would say to use 2 pieces of MDF but that will get heavy and be harder to keep flat. best of luck and welcome back.

Mark Shuman