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This is great stuff. My hat's off to everyone for the information.
The stuff your using uses moisture in the air to cure. The thicker you try to put it on, the longer it will probably take to set up. My experience with it was that it took more like 3-5 days to set up hard enough, deep enough to not do the seep, sag and bubble routine.
There are commercial compounds which are polyurethane based, which consist of 2 parts that you mix together. They have a variety of expansion rates, hardness, spongeness and so forth. They typically have to be mixed in some ratio of weight, so unless you have an arrangement for it, it can become a exercise in frustration.
If you use the thinner cardboard, I have a suggestion, after you get the shape, but before you lay on the fiberglass. I would take a small amount of resin, mix it so that the catalyst is on the high side and lay down a coat on the cardboard first. The reason is that you might have a problem, otherwise, with the cardboard becoming moist enough to start to lose it's shape.
I had a problem with a replacement air deflector I tried to make for my 89 Honda Wagon. I had created what I felt was a pretty good shape for my back window air deflector, but when I came back to it, I hadn't mixed the resin hot enough and the cardboard got soft and changed it's shape, before the resin set up.
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