6mm or greater is rigid enough for spans of a foot or so unsupported. If it will bear weight (like snow load) I wouldn’t exceed 6" unsupported in the 6mm board. For verticals, support isn’t much of an issue.
Standard thicknesses are 3mm, 6mm, 12mm, 189mm and 25mm corresponding to Imperial measure of .125", .25", .5", .75" and 1 inch.
I’d be happy to send you some, but the freight transit time to Oz would rule it out for the Mik.
Since Bill dug this thread up did anyone mention to him about Gator Board , Sintra, Azek, Polywantacracker?
Thanks Jon,
We don’t get snow but do get some hail. How does it hold up to heat? It gets to 45c here.
I’d love to have some board, but it would probably be cheaper to drive to Melbourne. (@Dave_Taylor do I have to count petrol costs in picking up an item?)
Thanks: 
Even the cost of the envelope would break the Mik rules. Ever since the USPS dropped ground shipping and the world got rid of its oldest cargo ships during Covid, shipping from the USA is astronomical. Recently I was quoted $71USD shipping for a thumb drive/SD card.
Generally good. Unsupported over a wide horizontal span it would likely sag over time. In a vertical situation it’s pretty good. I work in the sign industry and we use it for outdoor signs. When coated with black vinyl and exposed to the summer sun, it can expand quite a bit. We had to re-make a large black sign that buckled at a seam and then, when it cooled, the seam opened up. We added a sliding overlap at the seam to handle the expansion and the customer has been happy now for over 5 years.
Another material, as Rooster suggests, is Gatorplast board. This is a laminated sandwich of polystyrene over expanded foam. They claim it to be for indoor use only. I put an unprotected piece outdoors where it gets sun rain, wind and snow. It’s been out there at least 4 years and is dirty and faded but still strong as new.
