A friend had something like this (two paint brushes taped to some boards), but it wasn't quite what they wanted, so I designed this for them.

It is used for making the little indent near the spine on books. To use it just put it on the area you want to have the indent and clamp them down for a while. (I don't know 100% how to do this so maybe watch some videos on it if you don't know how) 

Print Settings:

I designed this to fit on a mk3/4/4S print bed so if yours is smaller you can use the cut tool in Prusaslicer to make it fit.

It is also very big, so I would highly recommend putting some glue stick at all of the corners.