@lemat I have read that heat treatment of low surface energy plastics can improve bonding with adhesives. Pretty sure the heat lamination of boogie board components relies on this. Perhaps something as simple as using a clothing iron at the correct temp could achieve better bonding of XPS with epoxy and/or other adhesives. Seems risky without a lot of experimental trials first.
I still say cork sandwiched between fiberglass skins over epoxy pre-sealed, properly perforated XPS of high enough density; laminated and fill-coated with opaque white pigmented epoxy will make a board that is durable if not exposed to extreme temperatures.
The first properly perforated, pre-sealed XPS core (1.5 pcf) bodyboard I glassed with 6/4-6/4 FG and opaque white pigmented epoxy is still fine. I only use it as the top board for strapping down my stack of XLPE Paipos and standard Boogie Boards to prevent the straps from permanently grooving the unglassed, soft, closed-cell foam boards underneath. The last round trip was 28 hours of driving in the hot, summer-solstice sun (directly over head) with regular stops in exposed, asphalt parking lots for meals and gas/restroom pit stops (4 passengers). The top, white XPS core BB took all of the direct exposure to solar heating (radiant heat gain) and ambient air temp. No de-lams yet. (I would not recommend any XPS Core board be treated this way.). Add cork FG sandwich skins with opaque white pigmented epoxy — thermal insulation of the XPS core and impact absorption should be even better.
Important but not addressed in this thread, large air filled cavities should be inter-connected and vented so that internal pressure changes due to heating (especially) and cooling can be equalized.
Finally, XPS construction techniques should be careful and meticulous.
EDIT: Many years ago a Sways member recommended wiping down the XPS surface with DNA. Since then if I use XPS, I wipe down the XPS foam slab with 90% Isopropyl Alcohol followed by a wipe down with a distilled water wetted (clean) cloth after the alcohol evaporated — before I begin shaping.