I think I have solved my own problem.
It looks like you must be very careful when reinstalling the cover and push it in straight. I opened it back up today and found that the O-ring had been shaved. Must have happened during installation.
This time I slowly and methodically pushed the cover in just until I could snap the retaining ring in. Took it for a short drive and had no leaks. Keeping my fingers crossed.
The local Ace hardware made the new O-ring for a buck.