I basically agree with Robert. On my Boler
, there is actually an air pocket behind the Ensolite there, because it goes from being stuck to the roof further up, to coming "in" to where the door frame is thicker. Yours might be the same way.
In fact, if you do have that air pocket, I would say it could be a bit of a problem to put the fasteners on top of the Ensolite, because you would have to worry about pulling through the Ensolite (it might stretch though; I'm just saying).
Note that the locking nuts are not really different in terms of shape or size; it's just that they have a "locking" component to the inside of them (some kind of plastic) which makes them less likely to come loose. It's like using a lock-washer only you don't have a separate piece. You probably still would want to use a regular washer though.
If you are thinking about putting an awning
rail on (or if you already have one), this might be the time to think about those fasteners too, since they are in the same area.