This is a model of one of the corner protectors that appear on Leo's Brynhildr tome. Since the model is symmetrical, you only need to print 4 in total, no flipping required!
The model is based on the thickness of this book box with a thin faux-leather skin: https://www.amazon.com/dp/B00KSD2KEI/
The model may need to be tweaked depending on the thickness of your book since you'll want it snug.
This model should be sized at 45x45x8.5 when you import it but it may require some tweaking for best fit.
The supports that it needs are to hold up the point when printing. I recommend orienting it with one of the long sides on the bed.
This model was designed primarily using 123D Design as a CAD exercise. It was imported into Blender to add the raised lip, since I was unable to figure out how to elegantly do that in 123D Design.