Low cost material

We are machining trays to carry parts in our manufacturing process. They are approximately 12"x20". In the tray there are a pockets where material is removed down to about 0.050"-0.100" to hold the parts. These pockets are arranged in columns and rows. The problem we have is finding a low cost material which will not bow. These trays get sent through an oven that reaches around 200F. The removal of so much material coupled with the heat causes the trays to bow after a number of cycles. We have tried polypropylene (1/2" both stress relived and not), and ABS plastic (1/4") (both products in 4'x8' sheets). Does anyone have a suggestion of a low cost material that could maintain its flatness? We have looked into Phenolic but cost is too high (we need to make about 100 each of 10 different designs). We considered metal but it is too big a thermal load for our ovens. We also thought of adding a metal skeleton to each tray, but again this raises cost significantly.