My suggestion is to have several points for the camera to mount.
To keep the cameralens middle (imaginary crosshair) pointed towards the book diagonal center you need a flexible mounting position.
The bookpage has an 45 degree angle, so the camera needs to be fixed in an 45 degree angle to ensure that there is no (skewing/keystoning?).
So I suggest measuring the different book sizes, then measure the diagonal center of the books. (Just cut a paper the same size as the book, take a ruler draw lines diagonally) Then you will know how high the center is from the bottom of the horizontal plane of the base. Imagine a straight line (or take a long stick which I do) from that center upwards. Then you will see how high the camera mount should be.
The reason for taking the diagonal center of the books is that lenses tend to get more (blurred/out of focus, whatever)
towards the outside. You will have the best quality in the middle. That is the reason why you don't get raw pictures from compact camera's. By slizing the edges of the picture, you won't notice it isn't that sharp on the edges as in the middle.
So for different sizes of books you need different heights where the camera can be mounted to insure you take pictures centered on the diagonal middle of the book.
Now since the "New standerd build" has 2 columns which are fixed on a corner, there is still the problem that your cameralens is not pointed to the book center, so you will get a border somewhere around the edges of your book.
So to keep the cameralens centered towards the diagonal center of the book one should not only have the possibility to adjust the camera in height (z-axes) but also vertically (y-axes).
Because the camera has the ability to zoom, this will make sure the picture will be filled as much as possible with a page from the book. (x-axes)
So the angle your camera in which your camera needs to be mounted is always the same as the angle of the cradle
(total cradle angle degree -(1/2 * total cradle angle degree) )
(i.e. 90-(1/2 * 90 = 45))
So when you are building a cradle which is a 120 degree slope the camera needs to be higher on the z-axes to have the same angle. Some houses (cellars/atticks) don't have that high ceiling.
Then there is the problem of how large your cradle is. If you want to make the cradle as large as your largest book to be scanned but also your paperbacks then you will have the problem that you can not lower the cameramount below the height of the cradle.
Only solution for that is to have different sizes cradles and platens.
This morning I have found an article explaining much better, with pictures, what I am trying to say.
The only thing which is not discussed is the vertical (y-axes).Fix Keystoning via hardware adjustments