I found a free download for printable model scale rulers in O, OO, HO and N : r/modeltrains
This engineer scale has six different measurement scales on one ruler. Requires legal-sized (8.5 x 14-inch) pa… | Ceramic nonstick cookware, Printable ruler, Ruler
Printable Millimeter Ruler - Tim's Printables
Metric And Imperial Scale Rulers Download | Railwayscenics