You'd certainly need those brackets to make something that's rigid, going for a thicker extrusion would help. I don't have any figures for 45x180, but looking at the moments of inertia of 40 x 160 they're 53x & 731y, whereas an 80 x 160 it's 362x & 1232y - 6x stiffer across the smaller dimension and nearly twice as stiff across the larger.