Your 8mm twin wall (link #3) or my 6mm twin wall should be rigid enough to support that weight no problem as long as it's edge supported. It's pretty stiff material because of the channels. Heck it's used in greenhouse roofs on a 4 foot span between supports!
BTW, I used a new utility knife blade to cut the 6mm TW PC no problem. No need for a fine tooth circular saw or jig saw.
Perfect, thanks again. Just ordered.