I used a hacksaw, a dremel with the heavy-duty cut-off wheel and some small files
To figure out where to cut, I drew lines to cut along, on the outside of the extinguisher body. This includes a nice ink pen, an architects scale (tri-angle type ruler), and a piece of paper. The ink pen could have been a scribe, instead. A sharpened nail, or ice-pick would work for that
I went and recycled a few photos between steps, as I didn't take enough pictures during the construction
Step 1: It all started with Pachanka's Wokfi
And this, is a 24 inch wok I found at Goodwill for $7, mounted on top of a telescope tripod. Handles were removed and the thumb-fi taped and hung off of some string into the focal point
But before I could test this wokfi, I read yet another instructable for a yagi cantenna, and after following some of the links in the comments I decided to build this cantenna
Step 2: In hindsight
So this is what I started with, a Kidde fire extinguisher that was empty and had been cluttering up a shelf for several years now.
It has an aluminum body with a nice flat bottom, which I thought would work well as a waveguide.
It measured around 3-1/4 inches outside diameter. This is like the kind you might find in a car or around a kitchen. If it had a plastic body, I suppose it could be wrapped in aluminum foil, but I would've scrapped the idea and gone for some exhaust tubing or something like that.