I have a 3.5" hollow square metal mailbox mount. Long since lost the cap for the main beam. I typically put a solar light their. piece of pool noodle inside the hole to keep it straight.
well the combination of me replacing the mail box with a much much larger one and me wanting to make it look nicer I rezzed up a simple post cap that was tall enough to clear the mail box and had a receptacle for a solar light. So now I just drop one in and done and it looks fantastic.
Print upside down and no supports are needed. mine got knocked off while doing the second solid top layer but since thats inside the post its not relevant.
printed on my CR-10 in Push Plastic Bronze PLA. (very nice pla btw)
Very simple model in tinkercad. clipped triangle square bottom cylinder cut for the solar light.
my post has an measurement of 3.25" inside 3.50" outside so its designed for a smooth slip fit into that.