I'm not clear here on what “masks” for a ICC clear car sides would look like? Can you be more specific?
Are you proposing a set of stencils (four to a car plan) to block out the window layout? (would require a fixed back wire for spacing)
Or are you proposing pre-painted, cut overlays that would eliminate the painting requirement for the sides entirely? I would be nice if we had options to do additional PS/ACF plans to augment the four types ICC (Vane Jones) did, like the full diner to match the Kitchen Lounge, baggage dorm, 22 rmte, a 4-4-2 etc
Comparatively, as is I find ICC exterior pretty easy to mask and paint using the specified white glue method, I prefer doing the interiors using appropriately cut thin colored paper instead of painting them.