The only way I got a successful curve was to fully soak the wood in water, then curve it, then finish it - but even then on the box that would be opened frequently I glued a layer of leather to the inside so that when it eventually broke it would hold together.
Living hinges are beautiful, but definitely better for things that don’t open all the time!
That being said - oh yeah no on the 1/4" unless your notebook is more than probably 4" thick you want to use 1/8", and yes plywood. The one I did in hardwood failed shockingly quickly no matter which way I laid the pattern against the wood.
There’s a pattern here that’s been working for a couple of years - but we only open it once a month or so.