I'm building a Nutball RET (Rudder, Elevator, Throttle) flyer, according to this page at rcgroups. I think most people use EPP foam, unfortunately that stuff is not easy to source in Canada. The main issue is shipping cost - the foam is very affordable, but to ship something that large, no matter how light, costs a fair bit!
Many have gone the route with the foam boards from Dollar Tree, aka DTF.
If this works well, I will want to convert it to elevons like my Teksumo, so I can practice with this and not the Tek, as the small park near my place is just too small for the Tek.