I made a corner cutter jig for my bookbinding mom. It will accept board material up to 1/8" thick, and sets the cut edge 1/8" from the corner. It cuts the corners precisely and aids in folding cover material over the hard inner board. I can make other offsets if someone really wants them.
Check out more on my blog: http://kiltedcraftworks.com/2017/08/14/cutting-corners