Flexa Crystal Clear Vinyl 8 Gauge (54" Wide) 5 Yards Long | Double Polished Marine Grade Vinyl | Waterproof, UV Resistant, CPSIA Compliant | Flexible Plastic for Porch Enclosure, Windows

You want 16 gauge crystal clear marine vinyl. Not generally marketed by mil thickness. Something like this (usually sold by the yard, and you can get different lengths here; not the seller I originally used, but $8-12 / yard is around what we paid, so you might find it cheaper elsewhere): https://www.amazon.com/Vinyl-Marine-Waterproof-Resistant-Compliant/dp/B0FZVCTQ19/?th=1