It's not possible to create your own custom size in the Book Module under normal circumstances.
Some people have done it by modifying the files for the existing templates. Re: A method to create completely customized photo book page templates from scratch in Lightroom 5
It is very sad Adobe did not allow to create a custom book size. I've researched this a bit, and people is asking for this feature since 4 years ago...
All software allows you to create custom size documents... I really don't understand why people is forced to use just 5 book sizes?
I've seen the document you suggested, but it is quite complicated... unless somone is willing to create a free utility that automate the process or perhaps, God willing, the next update includes a custom book size... seems to be I'll be forced to use the just 5 sizes already included.
I wouldn't hold my breath. I did an interview for my blog with the engineer that worked on the Book Module. He did say they were working on a way, but he moved to Apple to work on the Photos app for macOS and iOS not long after that. It's all well documented internally, I'm sure, but not a priority given the time that's passed.