I would think you could put the PH up and between the V in the support and the canvas not being attached slide it on.
Still I can totally see it being easier to pull the PH fiberglass, flip it and screw down following whatever indication is left on that half, as opposed to putting it together fighting gravity.
Once that's done I would put it back and then guess- by guess I mean cut your frame, clip things in place until it looks right and maybe experiment with putting the top up and down. There has got to be some leeway, and it's pretty much vertical when you're finished, possibly slightly larger than the upper portion.
Baring better measurements, I'd guess and just make sure your attachment is inside the fiberglass profile when closed- which I'd guess if you just kind of follow the canvas shape it should be.
EDIT: went through old projects and pulled random pictures that may or may not help
hosted/jage/random_top_pictures/