Rob, et. al.;

I have only found high strength XPS via construction suppliers.

Here is a local to you supplier of 700 kpa foam

http://www.dctech.com.au/styrofoam-hdx/

There are other domestic XPS makers, like https://www.knaufinsulation.co.nz/ but I didn't see a 700 kpa product and I stopped looking when I found a local 700 kpa product. There are likely several suppliers in this density.

AFAIK, this is mostly used under concrete slabs as insulation. Warehouses and big box stores need it, as not only do you have a concrete slab, but forklifts of product need to drive on it.

| EPS does not result in densities high enough to resist vacuum.

EPS is junk. Also called bead foam.