I included 2 models for converting your speed square into a scribe. This will fit a pen up to ½" thick, which includes a standard sharpie.

The Scribe Pinch will let you use the flat portion of the speed square as the base and will attach securely to the vertical part of the square. (I prefer this version.)

The Scribe Slide will slide onto the flat portion of the square, leaving the “base” as a T-shape. On my Milwaukee speed square, it's a little wobbly when used in this orientation, but this will depend on which speed square you're using.

Regardless of which option you choose, you will need to print 2 M3 knobs. You will also need the following hardware:

• 2x M3 heat set threaded inserts
• 2x M3 25mm Socket head bolt (if you use my M3 knob design)

If you want to adjust the design for your purposes, here is the CAD link: https://cad.onshape.com/documents/c7d5a879c10324ee0b51cde0/w/7d68bd9e0c8ddf93d87ab632/e/b100e8376695e6642112209b?renderMode=0&uiState=6929a2a738af00e2b57008f5