It's designed to be a snug fit, and matches the height of the table as closely as possible. So printing with fine details is recommended. With PETG I've used:

• 0.4mm nozzle
• 0.2mm layers
• 100% in-fill (done for rigidity, but lower values seem to work too)

Tip: print top-side down so you can get the absolutely smoothest finished surface on what will be the surface in contact with your work piece.

It was designed in OpenSCAD, so easy to customize to your own sizes:
[https://github.com/hcooper/router-plate-openscad](https://github.com/hcooper/router-plate-openscad)