I would use the unassembled box colums made of cellular pvc around a
metal or pressure treated wood post. The cellular pvc can be shaped,
machined, and painted like wood but it doesn't split, warp, crack, or
rot. Just make sure that the ones you buy come disassembled because
you will need to connect the parts around the inner post. Most come in
two sections, each with two sides of the column.
Some of the cellular pvc components sold are already factory finished.
If you must paint them yourself, I would recommend a two part epoxy
paint for extreme durability. Nothing looks worse than high tech
composite materials with peeling paint.