The straight parts of the arch that snaps onto the mount–the thin plastic edges–have broken. I don’t see any other posts about this so I suppose it’s not a common problem, and so doubt an improvement has been or will be made.
I contacted Support to ask to buy a replacement door, thinking it’s such a low-cost bit of plastic they might even just send me one. And because this has happened and might as easily happen again, figured I’d buy a spare or two.
They do not send or sell this part. Because it was physically damaged it is not covered by the warranty. Because I don’t think it was vandalized and didn’t get a police report I wouldn’t invoke the Theft Replacement Policy. The only option offered is to buy a $27 battery door and throw away the rest of the “Motion Sensor, Mounting Bracket x2, External Antenna, Mounting Hardware (screws, anchors, adhesive, wire routing accessories), Drill Bit, User Manual, Security Sticker” (per Ring Mailbox Sensor | Alarm Sensors & Alerts | Ring).
I don’t think a 3-D printed one would be stronger in that area, it’s just a weak design. I’ll MacGyver it with VHB tape and/or zip ties.