How about this for a spoilboard idea: (Apologies if this is old news.)

Cut strips of 18mm MDF (or whatever you use) to size to fit the vacuum table/gaps between T slots.
Seal edges.
Construction glue the strips to the machine base/vacuum table.
Skim all surfaces.

There you have it! A completely vacuum sealed spoil board - with full access to the T slots and that isnt' going to warp/bend/lift up etc.

I will use this as normal until multiple skimmings leave about 10mm left, then just glue new 18mm sheets on top. This process can be repeated ad infinitum - when your skimming reaches the glue layer just glue fresh 18mm strips, skim, and carry on.