When measuring clear dimensions in an area not close to an opening a meterstick often is not usable since the length of the stick usually is either shorter or longer than the area to be measured. For sure you can measure from both sides to a common marking somewhere in the middle but that requires two measurement steps and therefore is slower and less precise.

To resolve this I designed this extension. You unfold the foldable meterstick up to the size one leg shorter than the clear dimension to be measured and put the extension there. Then you read the length through one of the extension windows from the stick and add the number of centimeters printed at that window to get the total clear dimension.

For printing, I recommend using 0.2mm layer height and changing the color after the first two layers to emphasize the printed numbers. For sure you can also print it with one color only. The used material doesn't really matter except that you do not want to print measurement devices with flexible material for obvious reasons.

I also attached the OpenSCAD sources for customization in case your meterstick is thicker or thinner. Alternatively, feel free to directly scale in y and z (but not x) dimensions to make it fit.