There’s a few 3D printable corner cut jigs out there. I designed my own for three main reasons:

1. There’s a little ruler on the side to mark the width of the fold-over section of the cloth
2. It’s parametric (using OpenSCAD), so you can customize various sizes (spacing from corner, various thicknesses, size of ruler, etc.)
3. I like to model stuff myself because it’s fun.

https://makerworld.com/en/models/2000894-corner-cut-jig-with-ruler-for-bookbinding#profileId-2154412