A Mishnah Sukkah 1:1 teaches us aray, not a firm structure of 20 Amot!
B. Jews lived in ohalim in the wilderness. Did their tent walls not sway a bit in the wind? Isn't the sukkah a commemoration of same?
So what's the big deal if the canvas walls flap a bit - so long as they flap about as much as a typical tent?