I think I have the hot ticket.
I live on a busy road with no shoulder, so my box is literally a few inches from the lane of travel. I bought a Rubbermaid mailbox for $49.99 at Lowe's and mounted it with 6 screws to a pre-made 4x4 cedar post setup, also from Lowe's. I cut about 2-3 feet off the bottom of the post, drove some nails in the sides as anchors, then positioned the affair into a large round plastic flowerpot. The pot is then filled with 120 lbs of concrete [less than a cubic foot] and some potting soil [for the flowers]. I leveled a small spot along the road and hauled the junk-mail receptacle in the tractor bucket to the road.
I can't stop the low-bidder plow contractors from knocking over the box - pretty much every major snowstorm sees the thing knocked over. But, because it's flexible plastic, and all the weight is on the bottom, I can dig it out [or fish for it with the bucket] and tilt it up again. Also, it is substantial enough so the local drunks will damage their cars a bit when they hit it, but it isn't so rigid that I'll get slapped with a lawsuit.
Three years and it's doing great. The only downside is that I can't get flowers to live in the pot; not enough water, too close to hot asphalt, too much exhaust fumes. I may try cactus.