|Many thanks for this. I have sent Finnfoam an email about whether they have an Australian distributor and, if not, would they send some.

HIGHLOAD 100 and FOAMULAR 1000 are equivalent Rob. Might be more available.. Years ago groper had a local source.
https://www.boatdesign.net/threads/extruded-polystyrene-xps.24213/page-3

I sourced it from a concrete company that puts it under slabs. Afaik finnfoam does not export to the us. I have a link where they say these others are equilivant-ish.

|what 10mm FF700 would cost now.

For reference, 2'x8' sheet of not finnfoam, (~1.5²m) 3" (75mm) is ~$100USD for a single sheet. Cheaper in bulk. You might have to build that neat foam slicer.

If you do deem it acceptable, it would be neat for a builder to get a stack at the bulk price, and cut it down as needed. Instead or ordering the thickness, and hoping that it is enough.

|> I was able to get hold of him, and he offered to answer some questions!

Good job! Thanks Björn.

|> Now, I did not get that far, but I made some tests by hitting with a hammer and found that XPS had the ability to recoil - there was no delamination. H60, on the other hand, was very brittle and unable to spring back, causing delamination of the skin. (Mind that divinycell is available in higher grades than the H60 which I used.)

¯\(°_o)/¯