I've once build my own kayak and battled with the same thing , I've used normal waterproofing with membrane to cover the poly and then fibreglassed over that. The waterproofing is a waterbase and works nicely with the poly, easy to clean with water asswell. So if you cover you polystirene with waterproofing you can go over that with anything. Waterproofing is also relatively cheap. I would suggest the use of a membrane too. And , important to wait atleast till the waterproofing is dry properly before putting anything over it. Look I'm pretty sure if you put two coats of waterproofing on ,and give enough time for curing between the coats that it would be sufficient without anything else. When waterproofing cures you can fill the container with water and it will stay well in tacked.
Hope this helped