Decent bubble level for the money - U.S. Optics Rail Mounted Fixed Bubble Level

This isn't my favorite bubble level, but I wanted to stay away from tube-mounted levels for this build, and there was no ring-mounted option for my application (Trijicon Cantilever Mount w/Q-LOC). It suits its purpose, but I may end up swapping for the version that folds to the side as this one sticks out a bit more than I like with the protective cover screwed on.