There may be some useful plans/ideas on these sites to incoperate into your design.
http://www.nar.org/competition/plans/rocketglide.html
http://www.f4bscale.worldonline.co.uk/plans.htm
Glancing over them reminds me of something from the old days when I was into R/C planes you'll want to keep in mind.
Make sure your wings have a high dihedral angle (so they look a little like a V shape when you look from the front of the plane.
This gives the plane a tendancy to right itself and fly level at the cost of manuverablility. But since were building electrics with limited controlability, manuverability is not an issue.
Many of the glider plans on those site use a sites use a split-dihedral design with a level center part of the wing and angled wing ends. These is favorable for long wingspam craft since very high dihedral angles compromise lift as well.
\__|__/ <- sorta like this