Looking at other designs on youtube I found this one. Different layout approach which would be stiffer than your cantilever. Bit late I know, but any ideas from here?
http://www.youtube.com/watch?v=9zYrr4jNfWI

In terms of improving what you have - I don't think you'll get very far filling the back supports, or adding angle iron. You are looking for a big step change to make a difference. As Clive suggested either bolting to the wall or adding diagonals on each end coming forward to the front of the machine and onto the floor would be much better.

Can you post a picture of the whole thing now it is generally together? You might get a few more ideas . .