That’s fantastic. Is your 3D model on Thingiverse or anywhere else? I’ve been thinking about making exactly the same thing to use tripod mounts on the Moonlander.
Honestly, I measured the hole spacing as 19.5mm but it is definitely not quite right because it is tough to get all 4 screws in. I don't have gauge pins to measure it correctly, and the accuracy suffers. But you can get two screws in which was enough for my use.
I uploaded the Fusion 360 file so you can easily correct this.
I would also print it upside down from how it displays on Thingiverse. That way the mount <-> keyboard interface is as smooth as possible. (Well, if you have a smooth print plate, I guess.)