Crisp,
I believe some canoe skins have used sheet cork between (over and under) fiberglass skins.
This cork sandwich construction might make a good durable skin for your outrigger.
A little XPS prep work with a Wartenberg pin roller after sanding with 100 grit should significantly improve bonding. Pre-seal the prepped XPS with a thin coat of epoxy (then sand smooth with 150 grit) before laminating the skins. You can pick up a 7-wheel Wartenberg pin roller for about $15-$20 on eBay (get the neurological tool not the erotic toy).