Wow. This thread went sideways! OP asked for help on a license plate holder and the internet lawyers took off.
To the OP:
1. Are there any RV salvage yards in you area? You could check there.
2. Do you have or do you know anyone with a 3D printer? I have replaced/created all sorts of fixes for my TC using my 3D printer.
3. Go to your local hardware store (big box or small). Wander through the area where they have brackets and angle iron. I have found solutions for problems like this in the past by taking a gander through and finding inspiration or pieces to fix a problem. Make sure you take measurements and pictures of the broken item with you. It will save lots of time.
To the Internet Lawyers:
Why poke the bear? I have not heard of anyone that was stopped because they put the plate on the TC, but I have heard of people being stopped for NOT putting the plate on the TC and the police stop them because the police feel it is not visible. Putting the license plate on the back of the camper does not compromise any of your rights and only takes a minute or two.
I modified my license plate mount on both the truck and the camper so that the plate is mounted on (2) 1/4 bolts. The bolts are secured to the truck and the TC, so I just remove the two nuts that hold the plate on the truck, walk it over to the TC, put the plate and nuts on, and done.