I made this simple marking gauge block for use with a ruler from an old combination square. It also makes a handy depth gauge.

I got the idea from this guy's youtube video. He has a bunch of cool project videos on his channel and uses his "Linealanschlag" all them time in them. This one shows how he makes the original wooden version: https://www.youtube.com/watch?v=L-TO0nui0pc

If you have access to a table saw, you could probably make one of these things from wood faster that you could print it. Seriously, it's a block with a groove and a hole in it!

If you don't have power tools, printing one of these things might still be useful if you need to mark a line parallel to an edge of face. It might also be useful as a printer test block. The outside dimensions are 15 x 25 x 50 mm.

The holes are sized for the #8 Machine Screws and Wingnuts I had on hand. Metric M4 hardware should work too. It would actually be better with a hex head bolt (as with Herr Truppe's original) but I didn't have any handy. Even with a machine screw it grabs the plastic well enough to not spin while clamping. You might try that even if you go for the wooden version..

After I printed this one, I found it only fit one of the three rulers I had. The other two were too thick. You could scale it in that direction of your print turns out too tight or too loose.

Thanks for looking at my design!
Bob M

Edit: Added a second STL that should work better for a thicker (~2.4mm) ruler.

Edit 2: I did the original in Fusion 360, but since it's just a box with some holes in it, I've added an OpenSCAD script to let you customize it. Enjoy!