[herr_oberst]

You don't have to start from scratch. People give away suitable boxes all the time.

Wooden drawers or cabinets from a kitchen remod, home stereo speakers, flat-pack furniture. Maybe a decrepit old tool box or tackle box can be the basis with some imagination and creativity.
Find a suitable carcass on a craigslist or neighborhood free pile, and then augment it with your own ideas. Put a shelf or two inside, cut some nice planks for the outside to disguise the humble origins, and make it look wonderful. Then figure out how to attach the base to a post (maybe the post could be steel - 1 1/2 or 2" threaded pipe with the appropriate flange? Just an idea).

I wish I lived closer. I'd love to help you figure this out. I'd try to make it for as little cost as possible if not free, but by using creativity, make sure it isn't a blight in the neighborhood.
There's a guy that lives around me that turned an old gas pump into a parking-strip free-box for toy cars. His cost was elbow grease, not cash, and it looks fantastic.