The original plan of PVC sheeting Not really paning out. So going to do it the right way.
Tips.

Yep HDPE is good slippery as far as plastics go, most plastics joints should be able to get it in, I use a sign manufacturer/ plastic supply company in Newcastle to get mine, I think it comes via a Melbourne supplier.

There is also a recycled plastic that might be a cheaper option I have herd about

I built a bunch of boxes for a Vic ski resort years ago and used the guys at Dotmar. They were super informative about the products you can get. Can also use UHMWPE.

We used to always have ply underneath the HDPE but for some projects overseas we used thicker HDPE with no ply, just a few more cross supports. This worked out to be lighter.

The key for longevity is how you fix it to the rail/box/kicker. Dotmar can give you the details.

www.dotmar.com.au/index.php