Thanks for the help, guys.
"MaxSimultaneousOfType" seems to work better than the scripts, but it is still possible to start building more than one fortress simultaneously. However, it still works after you've (re)built your fortress(es) and you can't build any more unless you demolish all you currently have.
I've tried the combination of the two, but that just completely prevents you from building forts, regardless of how many you own.