RE: Need a big glider airfoil
EPP doesn't have a lot of it's own strength. To help things along without needing to get into complex spar systems that sort of defeats the simplicity of the EPP concept I would suggest a thicker airfoil at least for the center panes. Something like a Selig 4233 at 12% would be a good choice.
Actually any sailplane of this sort of span is going to require a complex and well designed spar system. The span alone means you're going to put a big load on the wing in higher G loaded turns and pullups. And if you want to make it a fairly high aspect ratio so it sort of looks scale like then you'll need a rigid skin on the wing to support it from torsional twisting as well.
At this size it's not a Cut N' Chuck project. Structural details to support the flight loads need to be carefully evaluated and planned for.