My insulation theory, take a look.

Gjac wrote:
You have a good approach to your project. The heat you feel is an exothermic reaction, all two part resins do this as they cross link or polymerize the way to reduce the heat is to spread the mix thin or make multiple mixes with less volume. I used to use 4 and 8 lb poly foam in work and mix it and pour a few oz in a paper cup and you can see what kind if expansion you will get. If you contain it and put too much into a closed mold you will increase the density and sometimes warp the mold. With all the large openings I would not worry about excessive expansion because you can tape the openings to contain the foam and cut the excess off with a sharp knife or razor if you use the lighter density foams 2-4 lbs, the heavier densities 8-16lbs or higher would require a saw because the get hard. You can also mix macro balloons into the foam to increase its insulating properties and reduce the volume of foam which would reduce the heat. Hope this helps and good luck with your project.

Since the polyurethane I'm mixing is meant for something other than insulating I don't have the specs for the density :-/

If you read my first posts in this thread I was thinking of inserting small pieces of rigid insulation and putting them in the inner walls before pouring the polyurethane but then I was worried that it could block the flow of the polyurethane when pouring so I decided that if I added some pieces I would add them immediately after pouring it.
The last thing I want is air pockets.