When you do the math, it's not cheaper. Even if the coroplast is free (likely won't be at the sizes you'll need) you're talking about hi torque servos, heavy duty hardware, a bigger battery, more expensive props, tougher landing gear and wheels, bigger engine mount, and so on. Unless the OP is a machinist and welder and has some scrap metal he can use, he'll be into the engine about $70 before starting it the first time. He can buy a .40 size glow engine ready to run just for what the parts to convert a weedeater engine will cost.