rivet?

First off some questions.

How often is the post going to need to revolve?

Is the post to be horizontal or vertical?

I ask this because the cheapest way would be to fabricate a rivet or an axel if the need to rotate is only for positioning or a low frequency use.

For a higher frequency you probably want some sort of bearing that can be replaced when needed.

Horizontal or vertical would dictate the kind of bearing needed.

If this is a low use thing the easiest way to get you there would be to select a 1/2 inch bolt that has an area clear of threads for 1 3/4" under the head. Cut off the excess length and drill a hole sideways through the shaft to secure a cotter pin with a washer or two between the pin and the material. A hair pin type spring could also be used instead of the cotter pin if you needed to disassemble it often.

Cut off the unthreaded part of a bolt and drive a few axle caps on it. Both can be found at Lowes, Home Depot, or any good hardware store...