The final picture in that post shows the problem. Imperfect focus. My theory was that the parallel parabola issue preventing full closing of the two halves of the form. When I tried it, the halves definitely did fit better, but the resulting mirror acted the same. The next possible culprit was Shaky Hands Syndrome. You can't freehand a perfect cut on a bandsaw, even following a line.
So I came up with an elaborate procedure that would let us use a router.
I'm not going to describe every little nuance of this thing, because seriously, it is much more complicated than it looks. I'm just going to point out the main idea.
If you recall, when I hold the mirror in place with nails it works great. So my theory was that constraining a springy object was making it closely approximate a parabola. So I took another springy object (the tan stick thing on top of the paper) and bent it to fit a parabola printed on paper. Then we used that as a guide for a router.
When I clamped the mirror into the form, it looked really good. There was not a single gap anywhere. But the result is indistinguishable from the freehand versions.
To my eye, the star pattern of the imperfect version looks like it is coming from short sections that aren't agreeing with each other. Like a faceted mirror. That would be easy to explain from a freehand form, but this latest version destroys that theory, at least as far as the form is concerned.
Yesterday I realized that uneven bending could be the result of uneven heating. Maybe some areas of the mirror aren't quite getting up to temperature. I was going to heat the entire thing up by another 10 degrees, but I accidentally broke the form. (Uh oh...it could also be shrinkage or other heat-deformation. I might be doomed.)
Also, I just realized that bending acrylic must be a technique used in industry (and art?). I should google around to see what I find. (I did that before I started, but more from the point of view of parabolas and less from the point of view of general acrylic forming.) (Update: Heh. Sounds like this guy had the exact same problem I'm having, 40 years ago. (picture caption, 2nd paragraph))
It's also possible I'm being a perfectionist. I'm going to have a 1/2" copper pipe running where the focus is. As long as all the energy hits the pipe, it isn't vital that it be pinpoint focus. Still, it's annoying that a thrown-together-in-5-minutes nail version performs so much better. It seems like I must be missing some really simple thing.
I actually do have an entirely different approach in the back of my mind. No oven needed at all...