We're in the process of updating our exterior, and wanted to update our porch posts. After visiting every home improvement store in the area, we found nothing we liked. So our choices were, build our own, or cover what we have. We chose to build sleeves around our current posts, since the depth of the posts would be much less than a 6X6 post, which would become a tripping hazard on our very narrow porch. More construction photos are at the link below.

Here is a link that might be useful: Porch Post Sleeves