Stewart does make its 4 way masking system, which I have, which includes adjustable masks top and bottom, and side masks which drop from the top to mask the 16:9 screen to 4:3. However, Stewart can custom make a screen with side panels which mask from side to side adjustably, but at a higher custom cost. You should e-mail or private message forum member Don Stewart about this and he can fill you in.
My main seating is 15' away from my 13' screen with some closer seating at 8' away. At this distance the picture is a little soft with VHS, LD, DSS material, but looks great with scaled DVDs and HD movies.