I did see a video from the US where a chap was building and putting in his own wooden mailbox post. He drilled a 3/4" hole down into the fence post just above the ground level, poured liquid creosote into the cavity, whittled a wooden peg to fit and tapped it in. Apparently the creosote soaks out over a couple of seasons and all you have to do is top it up every now and then.
Any use?