Hi SCRUBMONKEY
I built many SPAD planes without using the downspout square tubes you mentioned. I used aluminum sections for the fuselage, such as angles, channels, and square tubing. Our building supply stores, such as Lowe's sell them; the wall thicknesss is usually 1/16th inch. I flew Pusher Canard models with a 1" square aluminum fuselage for maybe 8 years. The photo below my name shows one of my Pusher Canards.