Stan,
That seems to happen a bunch. When we rehabilitated our
1967 Safari the 3 access doors had been replaced with make do patches. Our best replacements were made from parts from Vintage Trailer Supply including All Clad aluminum sheet. It's possible, but not likely, to find already made access doors that fit your trailer because dimensions vary. You will need pe opening dimensions so replacement fits tightly inside. For the rear panel I ordered one to be made and they still sent the wrong size using aluminum other than All Clad.
On each door I have attached a little chain just long enough to reach with a clip to remove it. Make the chain or cable short enough so that if the panel falls off it won't drag on the ground. Of course one must also carefully lock each door.
Jim