I drove many winters . . . I never put anything in my trunk, just used the proper tires.
I am confused at to why the OP thinks that his Blizzaks didn't work. If he got through 17" of snow, that is way, waaaaay beyond anything Blizzaks (or any tires) are supposed to do. . . . .
I second the above . . . I have driven RWD vehicles through the winter for a very long time and never bothered with weight in the trunk. Putting extra weight in the trunk is somewhat self defeating -- yeah it helps the rear wheels bite a little better, but you also have to push around that extra weight. Just run a full gas tank, it will do about the same thing and it won't go flying in an accident. That said, I did have an extra 50-lb bag lying around so I put it in there this year; don't know if it will make any difference.
I always wait for the roads to be at least partially plowed, would not attempt to drive in 17" of snow in a Mustang, nor in a 4x4 truck unless it was an emergency. In 17" of snow the car is going to be riding on top of the packed snow all jammed up under the car.