Finally found a week to make a new plenum.
Added 3/4" BahamaPly to original baseboard by rolling out a gallon of white glue and weighting,
Added 1" MDF same way.
Cut 4 zones by overlaying 5' x 5' and 4' x 8'.
Set threaded inserts in plenum. Screwed spoilboard to plenum with fiberglass screws.
Now can cut 5'x5' or 4'x8' without masking panels.
Now spoilboard goes on and off quickly without mess or dry time and no glue failure to cause leaky edges.
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