That looks a cool set 😃
These site smay be of interest mate - they convert inches to pixels for you (many covers seem to have a DPI of 300):-
A few people on here have bought old 3-disc boxsets from the local thrift/charity store (it doesn’t even have to have any discs in them etc) - and use that to take apart and learn how these box sets are made - as well as measuring dimensions etc. Some prefer 7mm slimline cases, or the 12mm spine cases - so sizes may vary depending upon preference.
Good luck with it!
“Why don’t you knock it off with them negative waves? Why don’t you dig how beautiful it is out here?”
“Why don’t you say something righteous and hopeful for a change?” - Oddball, of The Awkward Squad