Use the included STL or generate your own from the SCAD. The STL assumes 18mm width and depth for the caps, and 2mm clearance to shove the grabber tines through the gaps between the keys. The 1mm lip of the grabber portion may be a little too wide, making it difficult or impossible to insert between the keys. The edge can be easily trimmed to size with a knife, or the SCAD parameter "catch_thickness" adjusted down until it barely fits.

.scad also has an optional fillet to give some more strength to the tines, and optional text wrap (requires module write.scad).