Closed cell id prety firn. Oppen cell eill not hold water and it a good option not hard like closed. I just. Used plaind plywood sealed with epoxie pint and normal 2" firm foam with mesh for vetilationon the bottom.
I added a flat panel in the center with a embroidered logo installed them with
"Bolster clips" that allows the bolsster to be easily re moved good for storage.
Just an idea.
