Wouldn't a couple more channels make it adjustable enough? Maybe a flat along the length of the round dowel for infinite adjustment? Or do you want to avoid having to make any adjustments at all?
Yes, that would work, but it's a pain to adjust the book while scanning. I would like to make a system that is self-adjusting, because pulling the book out and adjusting mid-scan sucks and slows things down.
Maybe a threaded rod and and a pvc tube (5/8) over it will do the trick. The rod I can buy locally is 1 meter ( in length, cost arond â‚¬ 1,70. (+-$1,20) A piece of pvc, I don't know yet. Have 2 rubber bands pulling at the rod towards the right side, with heavy books it will gradually move to the right as the pages shift from right to left. I have no clue on how thin the pvc can be bought to put around the rod.
For accommadating the rod into the structure you will need some pocketing (shallow intendation) into your (ply)wood.
No sorry it needs to go all through because the rubber bands need to be on the outside to have a: adjustable strength rubber bands (more thick rubbers for heavier books, or more thinner ones) b: a (winged) bolt to put on either sides of the rod to give some extra friction if the book is not so heavy but stil thick.
A washer against the wood, then the rubber bands and then the bold for holding the rubber bands in place.