My take on a zero clearance guide rail for your table saw, or router table, etc. The guide fits slots 18.5 - 20.5(ish), and is adjustable after being mounted to your jig. (Provided it's mounted in the right orientation.) This model is 300mm long to take advantage of the bigger bed on my Railcore II printer.

You'll need four M3 x 6mm button head screws and nuts to assemble, as well as three #6 round head wood screws to mount to your jig.