Marinebeam makes some excellent navigation lights. However, for some inexplicable reason, they chose to make stern light (and some others) not compatible with standard (Aquasignal?) mounting scheme. It is not a drop-in replacement and some sort of a mounting bracket is in order.
The bracket was printed in black polycarbonate on a TAZ LulzBot printer in 2016. LulzBot struggled a bit with PC but managed to print it. It was mounted on a pushpit of Ericson 32-3. After years of use I saw no visible deterioration. 3D model (mb_stern_light_bracket) can be downloaded from my folder on tinkercad. By no means this is a professional design but it worked for me and was a fun thing to do.