Wow your unit is larger than the tape area down? What did you buy?
I have seen some command installations in w124's
http://www.jensmitbenz.de/command.htm
They look pretty nice - although it appears that htey had to overhand and/or angle the unit over part of the shift/console wood. I don't know if you have considered that option but look carefully at the photos on that page. Perhaps that is an easier option.