yes, I would advise the same thing: foam
If it's clark foam, choose a 80kg/m3 density ( more shocks on a kite than a surfboard)
perhaps it's a bit far from your country, but F Pacou, french shaper for "Wave"s" makes special cores for your needs:
here's his blog
http://waves.over-blog.com/
ask me if you need to contact him in french
++