Spar varnish for sign frame

We have new house numbers assigned by the 911 commission in our area, and I purchased a house number plate from them to put out front. Formerly, I had a LARGE single digit numeral on a plank that I crafted from 1-1/2" thick Cedar.
The new plate is auto license size (6" x 12") and consists of a reflective plastic backing bonded onto an aluminum backplate. I feel as a woodworker that this should be encased in a frame, which should keep the plastic from peeling off after prolonged exposure to the elements. What I have in mind is a plywood sandwich with a cutout slightly smaller than the plate, which I would finish with a few coats of Spar varnish. Any ideas from someone who has done a similar project?
Thanks,
--
Godzilla

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