This Brilliant² 1-Slot Variable ND Filter from Lindsey Optics is only 6mm thick and fits into most standard 4 x 5.65" matte boxes. Made with Shott B270 water white optical glass, it's ground flat so distortion won't be introduced into your image and then undergoes a multicoating process featuring a multilayer, antireflective coating to dramatically reduce flare, stray light, and ghosting.
Constructed with two circular polarizers, unlike many VNDs that use two linear or one circular and one liner polarizer, the Brilliant² 1-Slot Variable ND Filter can be inserted into your matte box tray with either side facing out, as the circular polarizers are assembled with the quarter wave retarder facing out. The 0.8 MOD gears on the top of the filter tray allow you to rotate the VND to change the density from either side of the lens. The tray has an integrated 1/4"-20 mounting hole for attaching a motor.