Well, here's so far.
a- I put two pieces of paperboard in like that
b & c-and squirted the great stuff in. These are views from the side and top.
d & e-After a few hours, when it'd somewhat stiffened, I thought I'd remove the cardboard so it wouldn't stick so bad. Maybe I should have sprayed the paperboard with PAM or something.
Partly (the drier area) came off better, I scraped it with that spatula, and the wetter area is where the paper stuck. And there was a hole in it!
f-I squirted a little more great stuff in, smoothed it with my (gloved) hand, and I'll let it dry overnight. Tomorrow if I can get to it before work, I plan to scrape off the paperboard on the other side, and trim it down more like I want it. I will update as it goes.