The outline is just a product of using the combination of a compass and tape measure . Aside from this, I tried to stay away from what I perceived to be mistakes in commercial designs. As to the carbon fiber for material, sure alu is heavier but it is still pretty light. The alu sheet is quite thin. I was afraid it might deform and get in the way of the crank, but after I gave it a bit of arch it has been holding its shape in a set it and forget it kind of manner. Overall, my guard could be a bit more robust than many commercial guards I know, but some customization might be unavoidable under the best of circumstances to get this level of fit I now have.