Product review (4 stars): Don't bother cleaning them, just replace them.
I've tried scrubbing old drip pans, soaking them in de-greaser, and dipping them in every caustic chemical I can find.  I've tried boiling them, scouring them and using abrasive cleaners.  Sometimes, it is just not possible to clean old drip pans.

These fit my Frigidaire stove perfectly.

Cleaning isn't fun.  Just buy new ones.