Hi Jim,
Not that it's much help but plastic material curves because it is getting hotter on one side than it does on the other or alternately is cooling down quicker on one side than it is on the other. It is probably quite easy to adjust the volume of the air assist for the top surface but keeping it cool on the underneath is an issue which I have not yet found a suitable way to resolve. (when cutting acrylic it sometimes bends upwards slightly, it never bends downwards). I think re-design of the cutting bed may be the only answer, perhaps your question will through some light on a solution.
Tweakie.
CNC is only limited by our imagination.