Teardrops n Tiny Travel Trailers

The time to install the Roof Skin on my Astroliner is quickly approaching. And with that in mind I would like to get as much feedback as possible from the Pro's on this forum. I vacuum bonded the Filon to the walls with epoxy. Which worked very well. But then again the walls were laying flat on the floor and bagging them was easy. Not so much on the roof. It has been suggested by others here to "Float" the Filon on the roof without gluing it down. I would like to get some pros and cons here on the proper method before I install a $600 piece of Filon. My fear is if I float a piece this size that it might move around going down the road, which I understand is the point to some extent. I just want to be sure I do it right. The Roof is 7'5" wide and approximately 20' long. I attached pictures to get an idea of size.

If you float the skin without a sheet of some thing behind it ,You will lower your roof strength.
Just some thing to think about.

I'm sorry, I should have specified... The next step is I will be putting on 1/8" Luan. Then, the 1/16" Filon.

I just used contact cement for the roof on my Filon covered build last summer. No issues so far. I'm wondering if this "expansion-contraction " thing is not such a big deal.

How did you apply the contact cement