_v3
 - Strange things happen: last week I ordered the third ruler, and what shall I say, in fact the new one has the specified measurement of 13mm width.  

Improved Version _v2:
 - The boreholes have shifted further inwards.
 - No melting inserts required anymore, M3 nuts are recessed into slots on the side, see picture

_v1
Just like I had a marking gauge for my larger 30mm ruler, I also wanted one for my smaller 13mm ruler.  A quick CAD session with my favourite CAD system OnShape, resulted in a realy neat and handy gadget.

Top and bottom are hold together with 4 M3x16 screws and 4 melting inserts. The stopper is made with a M5 screw and a copied star grip from here: https://www.printables.com/model/1518856-star-grip-hexagon-screw-m3-m4-m5-m6-m8-m10-m12/files (thanks a lot Alex). The nut is recessed into a slot in the lid, so it holds better than a melted insert.

The ruler is a Helios Preisser Stahlmaßstab, 150 x 13 x 0.5 mm (actually the ruler is only 12.3mm wide).