I cut some poplar (smooth grained hardwood, cherry or maple would work just as good) to 1/2" x 3/4", put a slot in it with my thin kerf table saw blade which happens to be just a hair thicker than the plexiglass I bought, then framed the bottom and sides of the marquee opening and cut a piece for the top. Once the plexi is slid in place, the top piece just slides on and is a press fit. The bezel around the marquee is only 1/2" and I only had to overcut the plexi by 1/4" all the way around. The smooth grained hardwood is both strong enough to not split or warp even when cut down to 1/2" and smooth enough to paint to a perfect finish.
I think it took me maybe 20 minutes to dimension, slot, sand, round over the edges with an 1/8th" roundover, cut and glue and nail into place (used 23ga brad nails, love that little nailer). Virtually no filler was needed because the tiny brads barely leave a hole