You'll probably have to move up sooner than you think. 41 qts aren't really that big and retics are active snakes. Even a pure SD male will eventually need something along the lines of a 4x2x2 or larger. A lot of your options for larger snakes really do best with a bit of vertical space and room to climb and move, even as juveniles, so a rack is pretty limiting.
My SD male is in a 48"x30"x18" Boamaster and he uses all of the space. He is very active. If I absolutely had to put him in a tub long-term it would be either a 74-qt under-bed tub or a VE-175 Christmas tree storage tub.