The undershield attaches to the 500E by 8 or so screws that are screwed into small metal receivers, which press into pre-defined points on the bottom of the car.
The receivers for the screws can strip out and are very inexpensive to replace, as are new screws.
There are no special cross-bars or other hardware requires to mount the undershield — just the undershield Itself, and the screws and their receptacles. You should be able to find the part numbers elsewhere on this forum, or look it up in the EPC.