Looking at all the installs, it appears, most people use screws to secure brackets to roof. A couple used well nuts or molly bolts. This attachment is more secure, especially when no rafters are available. Can the styrofoam insulation behind the plywood inhibit or degrade this attachment method?
Bill Rhino has a good video installing a panel with z-brackets. It doesn't look like an optimum design. You can't remove the panel without removing the roof screw & bracket. There's not much of an air gap between roof & panel either.
Sal