Hope this helps. Here are a pics of a C3 size L on fitted to both left and right boxes. In the left box, there are more than 3 inches and 2 inch clearance on the front and top of the helmet.
On the right, you need to tilt the helmet and the helmet front should be on the right side. It can fit most full face helmet, but I have only tried with an L size.
See the pics.