Looking for recommendations.

I have a very large shopbot with a vacuum setup that I need to be as close to flat as is possible. The cutting surface is about 110" x 300". On the frame is a layer of plywood biscuitted together. On top of that is 1" plastic(HDPE I think) with grids cut into it for the vacuum. Next is the 3/4" Trupan spoilboard. Pretty standard as far as I know. Ever since day one I have had issues with flatness. Usually starts off pretty flat after surfacing and gradually degrades. Sometimes slower and sometimes faster. The nature of our material(Plasdeck.com look us up) requires it to be cut at a consistent depth(within 0.005" would be nice) for fabrication purposes. It has come time to replace the Trupan and we are looking to rehab our setup. We will be checking for gaps between the layers and debris from screws in the plastic. Just about anything we can think of. Any thoughts on things to look for here would be appreciated.

Now, all this being said, I am open to rethinking this design even if it seems cost prohibitive. Are there any resources to look into or any professional I could hire to accomplish this?