Well, she actually decided this a few years ago but I only recently got around to getting it done...
I came up with the design myself. It was pretty simple and didn't really require much Shopsmith'ing, other than making some 45degree cuts in a 2x4, and cutting two 4x4 pieces to length. Here's a picture of the mailbox:
The vertical post is a full 8' length of pressure-treated 4x4, sunk into the ground about 30" with a liberal amount of concrete to set it. The 30" hole was dug with a 2-man power auger, which is a story in itself...
I actually set the 4x4 post in a vinyl 4x4 sleeve, then poured the concrete around that. This way, if this post gets damaged by an errant side view window or a wall of snow thrown by a snowplow, I can pull it out and slide a new one in. I filled the hole with about an inch of sand first, then snugged the sleeve down onto the sand. This ensures that any water inside the sleeve has a drainage path downward, and is not sealed up by concrete.
The horizontal cross-piece is a 23" length of 4x4, snugged into a 1" notch cut into the vertical post. I made this notch with repeated runs of a circular saw, then chiseled out the bottom. This was the first time I've done that, and the notch came out smoother than I expected!
I used a 36" Irwin Quick-Grip clamp to hold the horizontal piece tightly into the offset, made sure it was square, then used 3.5" coated deck screws from the backside to attach it. I drilled the holes through the vertical post slightly oversize to ensure that the deck screw threads didn't grip that piece well, and pulled the horizontal piece into the notch instead.
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