Take the clamp and place it on top of the fingerboard approximately there, where you would need to have it placed.
Look from above and see if the clamp has the correct width.
The clamp is slightly larger
Take some cork (shipped in the box), cut out a part and apply it to the clamp
The clamp is slightly smaller
Consider removing parts of the cork or make it thinner with a razor blade.
You can also carefully bend the clamp to adjust its fit.