I wanted to mount a German rack taillight (with the two bolts 50mm apart) to my rack bosses. So I made this parametric OpenSCAD model and put it on Thingverse Customizer:
You can change the inside width of the mount to fit your rack bosses, the height of the light, and the thickness of the part to fine-tune it. Then you can download the file to get it 3D printed.
This is designed to fit on barrel bosses - if you have the washer style you’ll probably have to factor in spacers. If you have bosses flush in the seatstays that don’t have parallel faces then this might work depending on how flexible your material is.
I haven’t printed one yet, if you print one let me know how it worked out.