I would use honeycomb core with thin plywood skins. Very light and easy to make. To match a crown is a little more difficult but not terribly. Edge finish can be either epoxy putty sanded flush or a wood insert. Wood insert would be easier on a flat panel. What is the thickness where the hatch sits on edge? A drawing or picture would be a big help. We have the honeycomb in grid scored which can be shipped UPS, the plywood you can get locally. Epoxy and fillers for glue too.
I'm still a little fuzzy on exactly what your after, so a picture would really help
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