It's not clear if the protection you're looking for is also supposed to achieve the "locked off" feature or simply protection. Also is this intended to be some kind of semi-permanent mount that has to survive heat, cold, rain, humidity, condensation, etc. or only for a couple hours at a time during a game in good weather?
You could simply use an acrylic or polycarbonate box. They have slightly different impact vs scratch resistance tradeoffs but either would probably work (there are various brand names like Plexiglas, Perspex, Lexan, etc.) You could mask off parts of the box to help prevent reflections and/or use a lens hood so that the lens is pressed up against the box so as not to pick up reflections.
You could use an underwater housing but that will likely be as expensive as the camera itself. There are also special enclosures meant for security or hunting cameras but those cameras tend to be quite small compared to a BMPCC4K so it might be hard to find one of an appropriate size.