My '91 300-SEL (W126) car has an encapsulation panel. Your W126 should have come with one as well.
In my case, it's a large plastic panel roughly 3ft x 3ft that attaches underneath with 4 small screws that have an 8mm head.
I wasn't able to locate a photo here or at other web suppliers. It may be a dealer item.
Some garages and dealers feel compelled to discard them once off for whatever service is being performed. If yours is missing, this is likely what happened to it.