Buy the door here: https://ausgeknipst.de/search?q=canon...
As those Canon A-line battery doors tend to break, we constructed and 3d-printed spare ones. Suits the Canon AE-1, AE-1 Program and Canon A-1.
You will need the long plastic part and the screw that is attached to your old battery door in order to use this part.
You can also check out Fix Old Cameras video on how to replace the door, if you need to remove the Top Cover:
• Fix Old Cameras: AE-1 Top Cover Removal / ...