Porch panel repair advice sought

Our front porch is a door with a glass panels in wooden frames either side. One of the panels is rotten, I plan to replace the whole thing (sometime!) but for now I want to cut out the large rotten panel and just replace with new wood. Would appreciate suggestions as to what material I best use to fill the inevitable gaps there will be at the joins. There used to be a thing called "Plastic wood" - probably there is something much better now? Thanks

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