you can buy a large poster board or you could paint a section of your wall?!?
I just bought a poster board and it worked wonders for getting a crisp background. And yes you could just tape the bristol board to your door assuming its long enough. you are going to have to punch a hole in the board for the nail to hang the bag so make sure you measure to see where the nail needs to go first.
I had too much trouble with fotofuze so I personally don't recommend it, but maybe you will have better luck with it.