I would skip the roof skin, and use 1/4" ply for the exterior.
I milled the strips to 3/16", about as thin as I can do while keeping strength for handling.
If I did the roof without sheathing, I would mill 1/4" for extra strength.
You would have to be sure that the foam of the roof, is smooth and fair, no gaps, so that the glue would get a good bite to the strips. I might even fair the joints in the insulation with small sections of 'glass. (good practice area!)
'Glassing of wood for clear coat is pretty easy. You just have to be sure to get the 'glass in full contact with the wood, otherwise, you can get bubbles or weave that will show up in the finish. No body filler here!
Finish is not too bad either. Four coats of a quality spar varnish with sanding between will give a nice look with good protection that shouldn't need re-touching for three or more years.