I didn't read the whole thread but Lye (NaOH) will eat aluminum very quickly.
If you have iron heads you could mix some powdered drain cleaner with water into a thin paste and use this to disintegrate the remainder of the aluminum heat shield. Just be sure to wear gloves and a face shield-chemical burns are no fun. Cleanup is as easy as rinsing with coca cola.
Nevermind, I went back and read. Specialty shop seems like the best bet.
On a side note, you walk with a limp don't you?
