Overnight everything dried and hardened. Last evening, while it was still setting up, I gently poked up at the cardboard web sections that had sagged from underneath. The Structolite was still soft enough to be flexible but firm enough to hold its shape so some of the saggy areas look OK now. I'll have to go over some other spots and holes but overall I'm fairly satisfied with the way things look.
My feeling is that Structolite is good stuff, very inexpensive when compared with Hydrocal or plaster cloth, and it's rough texture creates a pretty realistic looking base. However maybe it's not as suitable to being applied over a cardboard lattice as some other mix. It takes so long to set up the cardboard loses its strength because it's absorbing all the moisture from the paper towels over a long period of time. With something that sets more quickly the cardboard wouldn't absorb as much and would have a better chance of holding its shape.