Cheap foam core board from the dollar store can do this size well. Fold, bend and use hot glue to assemble. Check out https://www.flitetest.com/ for a huge number of ideas, kits (if you want them), plans, how-tos, etc. Almost any design to fit your mission parameters available, and skill level in learning to fly it!
Easy material to make prototypes, very quick to build with, and super cheap to make mistakes with or replace a model as needed. Use clear packing tape from the Dollar Store to cover/protect it, and add a bit of strength (get regular, NOT heavy duty or extra strong). Get the Dollar Store foam board - better than from art/craft places which are heavier, costs more, and doesn't add any noticeable strength.