I have an idea which would work for a fixed screen. I might give it a go one of these days. Simply put: construct 2 rectangular hardboard strips of approximately 16:1 ratio (where the long length is the width of your screen), cover with black velvet (or spray paint matt black), attach velcro to the underside end of each strip and the corresponding left and right top and bottom edges of your screen and .... bingo .... detachable masks.
And you can do the same for 4:3. This time the masks need to be in the ratio of 4.5:1 with the longer dimension being the height of your screen.
Mathematicians can shoot me down in flames, but you get the idea.