Kill cactus -- I don't know if anything is already made that would fit your budget, but you could put something together that would hold your backpack. There are always the homemade bucket panniers that might work. But what about fastening a small piece of light plywood or stiff plastic to the side of you rack. If you left a couple of tabs sticking up on top of the board you could hang your backpack over them to hold it in place. I would bet you could do something for cheap and it would not draw potential thieves like something hi-tech.