Here's the first instance of a soft book cradle, complete with instructions on how to make it:
Velvet book cradle
And here's the site that sells the book pillow:
Book support bag / cushion
There's also the more traditional library cradle:
Acrylic book cradle
That last one does not have adjustable sides, so that would have to be dealt with. (There is another model that has front 'stops' on it to keep a book from sliding.)
The last cradle (I should say cost-effective cradle) was one made for book binders. A sewing cradle is very close to what's needed, though I think the sides may be a bit too steep and, in the ones I saw, the sides weren't adjustable.
I saw some of the more commercial cradles for digitizing and wish they weren't so expensive! The most common one seems to be flat blocks of varying heights, where one side of the book can be raised and lowered depending on where the book is open to. This would be especially nice for thick books.
If I understand the process clearly, the main concern is to keep the plane of the camera lens even with the plane of the page being imaged, along with keeping the book
steady. To keep the efficient flow ereszet has worked out, it's necessary to not have to readjust after each image, so those are the two things I keep in mind when evaluating the usefulness of the cradles and pillows, etc.
I really appreciate the work ereszet has done. It's amazing!
(And you're right, I'm only doing English books - with a few scattered French ones - and nothing earlier than the 18th century. After reading your list, I realize I have it easy!
)