OK.

Cut a piece of teflon to fit inside a lenght of large aluminum U
channel.  Make the U channel several inches across the bottom, and
several more up the side.  A single slot in the bottom of the channel
allows a couple bolts to pass through the teflon and hold the plenum
down.  It also allows the tension to be adjusted so that there's no slack.

Thought of that one right after I sent the last message.

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