Plastic Canvas comes in various colors, and is sold in 10.5 x 13.5 inch sheets. The hole sizes are rated in "mesh": the larger the mesh number, the larger the holes. The mesh number indicates the number of squares per inch. For HO scale, 7 mesh is a good size.
The material is soft enough to cut with a utility knife, and stiff enough to be self-supporting. It can be used for industrial windows.
windows from white plastic canvas
It can also be used for ladder stock.
ladders from plastic canvas
Plastic canvas was used on this sanding tower (link to article).
And it can even be cut into crosses for your cemetery!