The headlight looks heavy-duty, and might be fairly heavy in weight too.
For the bike shown, perhaps some truss rod supports might help, (in the mounting of a headlight just above the fender).
The light could be attached to a piece of flat bar (1 thru-hole), and with each end bent 90-degrees and drilled for two more fasteners.
Then the bar could be attached to the truss rods, via commercial P-clamps (or R-clamps, etc.).
The reason for the two bends, is just so the angle of the light can be adjusted just right, rotating about the R-L axis of the 2 clamp screws.
Might not work so simply, if the light fixture is too much wider than the separation of the truss rods.