Home-made, from scratch
well, first of all, use balsa and foam. leave coroplast for larger engines. too heavy unless making a simple pizza box type thing. if yu must use a material like that, at least use foamboard like the folding flyer with a light coat of polyurethane to seal it. will work for scaled down spad and be much lighter. size? with foamboard I would go about 300 sq. in., balsa about 230-250 sq. in. why? you can't get as efficient airfoil out of foamboard as balsa... although you can make 3 simple flat bottom ribs, smash foamboard every 1/4" with pizza cutter from 1/3 chord forward on top inside, and curve/glue it to the 3 ribs. works decent. use strapping tape for "spars". weight no more than 20 oz. on that engine. oil collection? hah! that's a good one! with a norvel the goop goes mostly on one side of the airplane, but that's about as good as it gets.
radio range at 500 feet is not nearly enough. it will go at least 40 m.p.h. do the math; how fast will it be out of range?
running wide open is fine. use only an ounce of fuel at a time at first.
again, if scaling spad use foamboard. but I feel balsa has better strength to weight ratio and shock absorption qualities than almost everything. and more versatile. you can make about anything from it. for instance, here is my latest scratch built airframe
by the way, it is the only 1/2a plane that I have that stays CLEAN! the pusher engine works good for that. you might consider a pusher configuration if oil is a big issue.