Interesting idea. And it should hold up well. I was thinking to try and get away with 3/8" for $150 / sheet (plain HDPE) but am likely going to bump up to 1/2" for $200/sheet.
Just worried a long hot summer is going to make it droop. The hottest I've measured with an IR gun is ~146F IIRC and that is significatly below the melt point for HDPE so I am hoping it holds up. OAL is 84", bolted to three crossbraces. rear about 10" in, front about 12", middle brace splits the difference. So about 30" spans in the middle portions. And my rooftop solar covers the rear main section between braces. And I intend to add a second fixed panel forward of the first, so much of the shade deck will itself be shaded by solar panels. So I hope to 'get away with' HPDE and never have to mess with re-doing the deck again. MkIV and done.
Got my last parts for my rear axle bearing and seals and oil change. Probably do that work a week from now, after a trailering run upstate. Not entirely sure the parts will fit and don't want to start the project now when I'm trailering on Tuesday. There's 8.5", 8.6, 9"soemthing ring gears and different axle housings and some commonalities and no online catalog gave me full confidence. So I made a best guess and am hoping things are right. Won't know until things are disassembled.
And I got my new CV axle. Got a boot tore up and slung in my original as a stopgap to stop the slinging of CV grease everywhere. I'd kept one of my originals as a spare, when I'd put the new axles in last year(?) After looking at repair and replacement boots, the tools needed etc, the hassle, the $30 premium for a whole new axle was the far easier and cheaper thing to do. So it's in hand and probably going on this evening after the heat breaks.