I just happened to have done that operation today replacing the screening cap on my '90 300TE 4MATIC. I know what you mean, it is a bit inconvenient.

I used an (I think 5mm) allen socket on my 3/8" drive ratchet. I believe it was with a one-inch extension. Two-inches at the most. That was just the trick for breaking it loose. Then I used the long arm of one of those Bodhous (is that the right spelling?) ball-ended allen keys to spin it out.

Same going back in, using the ratchet (delicately, given the small size of the fasterner) to torque it down.

If I were looking for a better rig, I would go with a 1/4" drive allen socket setup, which would be more appropriate leverage-wise and would probably fit a bit better as well.

Cheers,
sc