Thanks Phatfoto and baja.
I installed really good, nearly weightless, reflective heat insulation between the roof and headliner. I also injected soft expanding insulating foam, the kind that wont bow window and door frames, into all the airspace between the outer walls and the interior plastic facade. All the cans of foam I used, together weighed only about 10 lbs, and so just the foam by itself weighs less than that.
I also carefully cut, then glued, memory foam, to fill all the dead airspace in the back gate. It didn't weigh the gate down at all. The gate shocks still open it easily and hold it open. The big rear speakers are now nestled in terrific sound insulation, and there are no more cold spots at all. You can barely hear the stereo, when you stand right outside the back gate, even when it is way louder than you could stand it inside.
Now it should be warmer in winter.

So far so good on that too, because it stays warm inside, for hours instead of minutes after shutdown. It should also stay cooler in the summer for the same reason. I sure hope I don't find out I am FOS on that.
The van is definitely now very very quiet inside, and my sound system is way better than I thought it was. I would not recommend touching the roof on a hot day though.

I pity any bird that tries landing on it!
I am one happy "dude"

, because my work already seems to be paying off. Now if I could just afford
this turbo, a BEGI FMU, waste gate, turbo manifold etc, I would really be in business. It drives and handles really nice. It only has "adequate" power though. Better than stock. But I want more. Good daily driver low boost would be a big improvement.