I am actually refitting my old SplashDrone II with a PixHawk 4 Mini flight controller and found a very small OSD module for it. But as it is intended for race copters it is just a board without housing.
So I designed a housing for it and also ensured that all connectors are bullet proof by keying them. I used standard 2.54mm board receptacles and have cut them into shape. Small pieces of 1mm ABS sheet are glued to the receptacles for keying.
If using transparant filament you can still see the board LEDs.
After installing the board just glue the lower part onto the upper part by some drops of CA .
For a newer cable harness which also supports a HC-06 Bluetooth module I am using two servo connectors which are glued together, see photo. You can also use two BEC connector housings which are glued together with CA instead of my cut down board connectors for connecting video in/out to get a more simple cable harness there..
The Bluetooth module housing is available at https://www.thingiverse.com/thing:3881030