My first goal was that it stores easily in a closet or in the back window of a pickup, then I wanted to make it as cheaply as possible out of stuff I had lying around.
All you need is a SO-239, cable, some wire, and a bit of 1"/25mm PVC and this mount. I might post some revisions, but this one tests out at barely above 1.0 SWR on both my Yaesu 897d display and my Elecraft W2 tester.
See the photos for how to assemble. This fits 1"/25mm o.d. PVC pipe. If you have a different diameter of PVC, just resize the lower half of the mount.
Tension from the cable running inside the PVC tube (or other mount) helps keep the ground plane wires at a roughly 45 degree angle.
After you've picked your center frequency, cut the wires down to 1-2 CM above your desired length, then slowly trim them while checking your SWM. I got mine under 1:1.1 in a few minutes.
I used 1/16 stainless welding rod -- it's just strong enough to keep a bend and stay straight while not being too thick/heavy to manipulate with a pair of pliers.