I wanted to create a cheap and simple way to protect the glass lens of our expensive front cameras from flying debris. Though a reflective coated plano lens would have been preferred (very cost prohibitive), I settled on using a glass watch crystal held on by rubber heat shrink. The mineral glass is 25mm diameter and 2mm thick (a tad larger and/or thinner is ok), and internal reflections are minimal under most conditions. The tubing was heat formed around a steel socket-set piece matching the diameter of the camera, with a bit overhanging the glass to keep it in place. This produced a snug fit that won't slip off the camera until you want it to for cleaning or replacement. Total cost of materials under $5.
ps- Apparent in the below images, it has already saved my lens from a rock chip after only 100 miles of testing!