Maybe you could bolster the o-ring with some RTV, make it a bit thicker. In that case, I would hold the cover in place with some sort of contraption (when I did it, a piece of copper pipe worked fine!) and seal the outside edge of the lip and put the o-ring in. That would seal it and keep it in place, but you'd have to be absolutely positive that the RTV is dry and bonded before driving anywhere.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth