time to time I see large panels used for cool rooms going for a song.
Would they be any good for our needs ?

should be ok.
i'm pretty sure it's SL grade.

Trouble is usually too thin.
If you can find it in 200mm then hell yes.

Can glue it together but hard to shape well

I found some 2300x1200x100mm for $50.00
The guys has more if anyone interested.
Never be in contact with salt water.
Will be the basis for a possible foil board.

If you can find a Stratco insulated roofing installer or similar these sheets will come with just eps as covering pieces and large blocks to lift up for forklift handling & they are just throwing this stuff away maybe you could get it for free .

Is there any way to determine density by looking at foam ie does the size of foam bead represent density ?