Quite simply, I built it with wire.
I used some wire from an old computer-printer cable. One long one, bent at a 20° angle (of course) in the middle. Then I added a short wire, hooked on the long one at the mid-point. The wires are held down in the paper frame with Magic tape. It was quick and fast. I held the focusing mask to the lens shade with tape. Quick and fast.
And here's the proof:
Look at that lovely diffraction pattern. Just like a Bahtinov mask! But it is letting in more light. It's transmitting more light than the small Bahtinov that I tried.
Now it wasn't perfectly uniform or symmetrical. I wondered if it was because the wires were not perfectly straight. Or that the hook of the second wire in the middle was messing with the light waves. Or that my false star was too close. Or that my false star was an LED aimed to the camera as opposed to a false star by reflecting off a sphere.
Any how. I'm thrilled! Version 1.0 worked!
This test was all inspired by the talk on Stargazers Lounge forum.
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Version 2.0 will use a cardboard frame for rigidity. Perhaps I will mount the wires through holes in the frame so I can apply some tension. The short wire will be soldered straight to the long wire, without a hook. Oh yes, and a cone or tube to allow rapid insertion on the camera lens.
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Now I need to revisit software tools that can analyse the diffraction pattern.
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