My 10-year old canvas cover on my top deck is getting tired. The canvas is on a metal frame and measures about 18 feet wide and 22 feet long. For about the same or a little less than the cost of sunbrella, I can replace the canvas with a plastic material like Vycom UV Stable PVC board, Lexan polycarbonate, or Komacell expanded rigid PVC board. Some of the plastic materials come in 4X8 sheets, but some come in unlimited lengths by widths up to 100" for a more seamless installation. I would attach the plastic sheets to the top of the metal frame with screws. Silicone under the screws would make the installation water tight, although leaving the silicone out would still likely give fewer drips than sunbrella.
I've got no real experience with the project as you're proposing but some experience with something similar to some of the products..
I used four by eight sheets of a vinyl product for siding or skin on out boat. The good is it's tough and it cleans real easy. I also utilized screws for attachment to the boats structure without any sealers and it's stayed in place well without any leakage or water penetration.
Now on to the potential negative of the product... Thermal expansion and contraction. I got lucky in a way and installed my siding pretty much in thermal neutral weather temperature wise. But even so in warm weather with the sun shining on it the siding does take on the appearance of a marsh-mellow effect since the product is hard fastened to the walls as it expands with warmth it had nowhere to go but swell out away from the walls between the fasteners. Mines not to bad but it is noticeable but I've observed extremely bad results with others usage of similar materials when applied in cold weather like over winter when a lot of us work on our boats.
So what I'm saying is neither good or bad or maybe not even helpful but thermal expansion is a real consideration with such products...
Respect Our Outdoor Recreational Resources Leaving Only "Footprints in the Sand"
I put 4x8 sheets of FRP under my canvas. I did it to make it more weatherproof, to help extend the lifespan of the canvas, and to cool it down under the canvas. I screwed it down with self tapping screws with rubber gaskets under them - they are used to screw down tin roofs. Since my canvas covers it, I used foil tape to cover the seams. Just be aware that the framework for your canvas will not be square.