Thinking out loud:
The easiest way to get an accurate Z-axis distance from the bit-to-board, would be to actually cut the particle board Bed Cover with the Router.
Then, to conform the 1-sided or 2-sided Copper Bd. to that bed; we might want to cut some vacuum channels into the bed during the same run.
Then, when you mount a Bd. to the bed, just Duct Tape over the open channels. It will all be good, so long as you don’t drill the Thru Holes first. I figure that 600 mBarr of vacuum will push the Bd flat with 8 lbs / in^2 of force.
A typical turbine creates 800 mBarr, so maybe a Low Pressure Check Valve T-Joint between the Bed and Debris Vacuum Cowl, with maybe a Venturi Aperture to keep the valve from fluttering. These valve are cheap from Grainger, and provide an elegant solution.