I faced this same issue restoring 2511. I spoke with Steve Wilkinson and he can provide a rear panel. However, he cautioned that he usually suggests having the car on hand for proper fitment. I believe he has the panels made over size and I think any competent body shop should be able to successfully install even if some metal work is needed.
In my case we decided to just remove the entire rear panel to fix previous damage that needed full panel access for hammer and dolly repair.