Just for clarification, the thickness must certainly be much thinner for stencils… Perhaps 7-10 mil (.007"-.010") or .07-.10mm?
With a diode, color of the mylar will be a MAJOR factor. Water clear will be impossible and most lighter colors (white, yellow, etc) will be extremely difficult. Best luck will be dark opaque colors…ideally black.
Search here for keywords “mylar” and “diode”. I don’t think you’ll find many success stories. Acetate may be a better choice.