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

This is 3mm cast acrylic.

The design started with CardBox from Boxes.py

I made a few modifications, such as improving the lid design and adding holes to make it easier to access cards.

https://m.youtube.com/watch?v=oGAHh0GPKRA

Even after all of these years, the intro still impresses me every time I listen to it.