This head unit mount is designed to be used with a 90 degree DB-9 adapter (I used this one, but most should work: https://www.amazon.com/dp/B010V8HNR8)

Before assembling, I had to hand ream the 4 mounting holes with a small (~2mm) drill bit; once done, I used 4x M3x8 screws, which threaded into the plastic beautifully. I'm not using them, but if desired the two protective arms can be attached to the outside of the head unit.

The two mounting holes in the bracket are provided more as pilot holes; for my application I will be using self-tapping screws. I included the FreeCAD files, so feel free to modify to your needs (hopefully they are intelligible, I'm self-taught and this is my first public project lmao)