I had the same problem with my 1989 300E! Consider the following:

Another post suggested going down to the local auto parts store and buying a cheapie plastic cup holder ($2-$3) that was designed to hang from the driver's window. It would have a plastic extention on it with a hook! Break off the hook but leave the plastic extention on it. Slip the extention between the carpet and the transmission housing just in front of the driver's (or passenger's)seat. The cup should rest on the floor and is held in place by the extention wedged into the transmission housing. Works great! Holds even large Gatoraid bottle without a problem and does not interfere with my leg or driving position. Just a thought!