Yep, that'll probably work. Keep an eye that the tyre doesn't rub on the bolt and you don't get any stress causing the mudguard to crack.
A more elegant option would have been to get a thin aluminium mudguard bracket and have it mount underneath the brake caliper so it wouldn't catch on the metal block, the plastic mount was probably a bit too big to do that though.
A couple of photos with the caliper removed would probably have got you some better advice quicker though as that metal mount on the frame is uncommon and wasn't visible in your original photo.