Bring a printed set of the plans for the plane when you visit the sign shop. if they think you are goinf to make your own sign... they charge $20 for a 4' X 8' sheet of 4 mm coro. The 4' X 8' sheet should be close to $10.

If all else fails... yo can order the coroplast from Harbor Sales (harborsales.com) $50 worth of coro... shipping to continental US is free. (If its cut to go UPS @ 2 ftX 4 Ft... and watch the flute direction!)
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Its durable. you can sometimes take one of these coroplast planes, drag a wingtip at full throttle knif-edge pass.. pick it up after it cartwheels 50 yards... start the motor and fly again.