Glass vs. build up
Glass covered hollowed out blue foam might be an option to consider. One of my buddies built a C130 with 6' wingspan, powered it on those little GSW motors swinging a 4 blade prop.
It sands nice, take glass with some Zpoxy. You leave a 1/2" - 3/8" wall thickness with an oz of glass over it, paint it with acrylic and you'd be there.