Lamin8r a lot of them seem to use round tube curved to match the contour, i've done it that way myself but it don't give you anywhere to attach panels as its round and i can't see it being as rigid as square tube in triangles as if its curved it can still be flexed on the bend where as the square tube allows trim clip holes to be drilled into it. Not sure of the science of it all but my cars don't have any rattles like some glass cars.
Obviously if your doing bodies to sell you may have to go the round tube route as it is less time consuming.