I have read with interest concerns about cardboard bulkheads warping, especially in ship models, as the model goes together. I tested an idea that did not work so I will not bore you with the outcome. However... Here is the fix to eliminate any warpage in any model; follow the link and purchase a large sheet of Letraset 2000 mechanical board. It just happens to be 1mm thick. If you need thinner board, just delaminate what you need. This board is dimensionally stable. I have a supply that I have been carrying around the world for over 20 years and it has still not warped a bit. I had my supply stored in a room that suffered some water leakage; the board in contact with water suffered while the rest remained undamaged. It is more expensive than el cheapo cardboard, but when you consider no hassles, no headaches, it's a bargain. A large sheet will cost a bit over $7 plus shipping, more than enough for several models, depending on their size. Check your local art store. You'll save the shipping cost.