Finally, I contacted Tom Hanson, who now works at the MB Classic Center and who I know from MBCA. Tom is a real expert "parts guy." He recommended MB part number 1108850081 for this application. After checking with several sources of MB parts, I ordered one for $43. It isn't here yet, so I can't say much for sure. But a basic description of this plate frame is that is like a "tray" with raised and rolled edges. You set the plate in the front of this part, and the rolled edges form the "frame". I am not totally sure how I will secure the plate, but I do have some good adhesives. Supposedly, it isn't hard to form this part to fit bumpers that curve. There are holes in the "tray" part of the frame behind the plate for securing it to what ever kind of bumper mounting you have ---- or it wouldn't be hard to drill additional holes if needed. I'll keep you posted after the part arrives.
Thanks,
Richard