First thing I would do is make sure its not illegal, most mailbox posts and many towns/cities state what type of post you can use. They are meant to "breakaway" incase a car hits it or a snowplow. Ours has to be x amount away from the road X amount in height and has to be made out of a material that will bend or break off in an accident.
Was a case around here where the driver (who was at fault) slid off the road in the winter time and hit someones paver brick/concrete mailbox "post"(more like a foot square of concrete and paver bricks with the mailbox inside it all) the guy made to prevent the ol baseball bat from hitting it. Driver was OK but passenger was pretty beat up as the door caved in due to the homeowners "mailbox post". Tried to sue of course but i never ended up hearing the end of it...
With all that said, id imagine you could incorporate a front axle as the post, maybe triangulate some radius rods as a support, could fit an oil pan upsidedown over the standard steel mailbox as a cover (maybe 2 oilpans bolted together as a clamshell with mailbox in between? remove bottom pan plug to let water out...)
depends on what parts you have i suppose.
around me Ive seen some interesting ones, seen 2 fence post augers augered in the ground, cut off then angle iron welded across the top, RR switch flag and mechanism, seen a few where they took the standard mailbox and attached steetmetal cutouts to turn them into tractors and the mailbox door is the radiator. Ive seen the same done and it resembled a model T. These are usually out in the sticks where they dont have any type of mailbox ordinances as i mentioned above...
