Definitely usable but there is enough flex to be cautious of at the thinnest outside edge. The platic can be twisted/bent there without much force. Basically i would not have faith in the part to rebound back to the exact position to be perfect for moulding once it has been played with and mounted. As stated above I would actually attach it to the bumper or some solid template created from the bumper curvature to ensure its as close as possible. To JHMS point a couple millimeters is expected so not everyone is going to have the same fit, just comes down to getting as close to that 1-2mm variation as possible so the majority will not need to mess around with them to get the fit right.