I can not find 2" thick Styrofoam / EPS Sheets.
I’ve found 1 1/2 inch - would my oven work okay with this size?
No!!!.. You’re lucky!!
I’d prefer a 3” insulation layer vs. a 2” - but nobody carries 1.5”… So you lucked out!
Simply make 2 of each panel then glue 1 panel to the OSB board, then glue a second and equally size panel on top of that first panel. You will now have a total of 3".
And 3" of insulation will insulate better and be lighter to lift / slide the slabs onto the base.
Make sure you remove any film or foam before gluing the panels together.
Only use Gorilla Glue - all other glues melt the foam. Gorilla glue does not - just bonds the panels with freaky strength!