After a while of just sitting and staring at the PowerBook innards, I finally thought of an easy, stable way to mount it: 22 gauge speaker wire. I had a lot left over from the MAME project. so I cut a few pieces and threaded them through the grommet holes of the little swivel things on the back. No drilling or cutting.
The speaker wire basically acts as large twist-ties (except stronger and insulated).
One vertical wire, one horizontal wire, and one to hold the HDD in place.
The wire threaded around the back: