What you are trying to do, end up with a smooth entry and exit from the curve on a small radius is very difficult. The usefulness of springs and sand runs out quickly when the radius is tight and aesthetics are important.
I needed a similar bend in smaller dia. copper, which, I'm guessing is easier than al. The problem is that it is really hard to avoid kinks or material build up along the leading edge. I've attached drawings and whatnot for a universal bender of my own design. There are two features to it 1) the former for the outside is linear and follows the pipe around the curve (circular rollers tend to plow up material along the leading edge), and 2) the traveling roller is on a cam so that you can tightly pinch the tube between the two formers.
It has since been built and works fairly well, although it was only designed for up to 3/8 copper. It will bend an inside radius almost to the tube dia. No rep and warranty on the drawings, but I think they are pretty close. There is also a tube straightening mill I did as well. Both look real nice and sit in a drawer with a 10^-8 duty cycle
