Simplest (and cheapest) solution for your shoplight is duck tape. Yes duck tape (used to be called duct tape, when it was still legal to use it on ducts). Just run a strip down the length, then another tape to tape on the inside. You can spray paint the entire unit whatever color you like (I'm partial to flat black) including the tape, and you won't be able to tell it's only tape. You will need to use a primer coat before the final coat, or the paint won't stick well to both the metal and the tape. Since the tape is flexible, it works very well with the unit, forming a nice seal on the tank top, so light won't leak into your view.