A person who visited the BMW Enduro Park Hechlingen / Germany told me, that BMW does not recommend covering the radiator with anything. One of the trainers told him BMW thinks the radiator needs every fin in some riding situations. Well, I do not believe the radiator is really in much danger. At least I did not hear of radiator damages. Anyone? Instead I believe this looks a little poor:
I bought a cheap aluminium grill from a car parts shop, and used a piece of paper cut into shape as a template:
I use a self-adhesive rubber strip around the beak and the frame. The grill is cut to tightly fit into the window. It is held secured by two cable ties.
The fins point downwards, where I hope they pick up more wind flow:
