I borrowed a grinder from my Dremel kit and inserted it into my drill press. I knew control would be difficult since I would have to fit the oil cover in a wise and then move it manually. The biggest issue is that the grinder will bite and kick the piece away at one side of the working area.
Old and "new" cover side by side after I had ground the ear lobe remains away and created a flat on the outside of the "new" cover.
These pictures makes it easier to see what has been ground away
An aluminium spacer I had laying around (since I never throw anything away) with the same thickness as the modified cover in place
Next step was to make a clamp to retain the cover when the bolt was tightened down. I made a template from paper and transferred it to a piece of scrap aluminium I conveniently had laying around
With the aid of a drill, hacksaw and a couple of files I ended up with this retainer
Finished product. With more practice I should have been able to move the cover around in a better way to create a smoother curve around the edge of the flat I ground, but that would be for aesthetics as function would be unchanged. Since this is a rat anyway, I'm not too bothered - although it is only human to want to do better. Anyway, with zero cost and an hour spent it was good value.
Compared this repair to my first, failed repair attempt, I reckon it was a good upgrade.
With my mistakes explained and the documentation provided I'm sure you can do better.
_ --- or not 
- the bad just seems to last longer...
