Click here or on the image below to obtain an image of three reference scales to send to a photofinisher to produce a regular 4" x 6" photographic print, as pictured below (or print on your own photo-printer).
Photofinishing on photographic paper is a good choice because it has high contrast, resists yellowing and fading, has excellent resolution (no visible half-tone dots), and is fairly durable and somewhat water-resistant. It's also easy and inexpensive. Laser printing on paper or pressure-sensitive paper works less well; the grey patch often ends up with a half-tone printing 'grain' and paper can yellow with age and sun exposure.
Make your reference scale more durable by mounting it on a piece of old ruler or similar. I use a 10cm piece cut from an aluminum ruler, and have also mounted scales on a section of a wooden paint stirring stick from hardware stores. I use a thin spread of 5-minute epoxy to mount the scale. (Avoid contact cement, I found the toluene solvent caused print dye to bleed.)
A compact 10cm reference scale with a checkerboard for 2D scale, a circular fiducial mark for perspective measurement, and a grey patch for colour correction that is printable on a photo-printer or as a regular 4"x6" photographic print.