I wrote up a detailed tutorial (just half a notch above spoonfeeding) on waveguide antennas using Tetra Brik soup boxes. These used to be uncommon in the US, but now Swanson is selling chicken broth in them, and you can find the product in virtually any supermarket.
Here's the link:
Novilio's Tetra Brik Wi-Fi Antenna Tutorial
Not quite as good as a spaghetti sauce can, but you can make one (if you're careful) with a sharp Swiss Army knife and a soldering iron.
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--Novilio