I started snooping around on the Dow Chemical site, suppliers of both Great Stuff and blue board (styrofoam insulation board). They note that Great Stuff requires moisture to cure, so they don't recommend it for large panels. Maybe I don't care if enough moisture gets to the edges, they'll cure and hold the board in place right away. Perhaps the sealed in part will cure over time. Since I need a good bond for thermal insulation, there may be a better solution. I'll see if I can get other suggestions via the DOW site. As long as I keep researching, I don't have to get my hands dirty!!

BTW, I used Great Stuff a few years back to seal a septic tank riser and it worked great. Of course there was no lack of moisture around to cure the stuff!