Andy,
Doug's whatchamacallit does sit really flat on a flat surface. He doesn't try to make it hold the rear triangle in place. I think if a fixture rigidly holds a frame out of alignment it's probably even worse than poor heat control. Most people just tack in their fixture nowadays.

I made sure my fixture could use Anvil dummy axles. I have considered trying to get Paragon to sell me the ones that they make, but they probably wouldn't. They are very accurate, particularly if you use their dropouts, which are also more accurate than most.