Normally a pad of insulation was glued to the door skin and occasionally also to the inner side of the door facing the door skin. This stuff was about 1/4 inch thick and it was a mix of dense fiber paper like that used in egg cartons and tar. The pieces were cut by rule die cutters to the size needed. That's the best way I can describe it. I don't know where you'd get it today, although I did see where GM used a similar product as late as 1998 on a Pontiac I had to replace a window lift motor on. That product wasn't the same as the old stuff. It was more like tar paper but 1/8 inch or maybe 3.5mm thick. You probably could use 30 pound tar paper and laminate it using some waterproof construction adhesive (Liquid Nails Extreme). It isn't going to be any fun getting this installed in the inner door, though.

I would avoid any of the self-stick insulating stuff like MaxMat or anything resembling it. I doubt you'll be able to get your inside door clean enough for it to bond and the first time the sun hits your door it will come undone. Plus much of that stuff is just foil faced bubble wrap with a dual sided adhesive attached to it and sold at a nice profit.

If you could find a closed-cell foam (like neoprene) which was 1/8 inch thick, you could glue two layers of that to the inside of your doors and it might be easier to roll up and unroll once inside the door. I would not use anything thicker than 1/8 inch, though. Too hard to roll up. You could laminate that into 1/4" thickness by using two layers. Good luck.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end