Like you I am in Australia. I have an XL and about 6 weeks ago ordered unseen a sheet of 20mm HDPE (cost about 150 AUD). To say I was disappointed is an understatement, presently the stuff is cupped to about 12mm deep on the long axis. This material is really stiff and I am sure if I bolt it onto the Shapeoko it will deform the steel rails.
When I brought it home I plonked it against the garage wall and left it there in disgust. However I am sure it was then much more cupped than the present 12mm. Perhaps in time it will straighten and the cupping was caused by the way it was stored by the supplier. One can only hope.