I'm not sure if the dash in your 420 is the same set up as my 300SD 126, but I did make the dash removal tool out of a coat hanger. I looked at the tool in the MB manual and fabricated mine. It worked fine.
Then again, my vehicle is a little older, and the dash is probably not as tight, but it did work, with out paying the money for the factory tool.
Best of Luck.