This post shows a variation on a rounded drop spine box that I make. I try to not have a break along the top and bottom of the tray so it has more of the look of a book. To do this on the outer tray, I extend the top and bottom board piece past the tray base and then cut a round shape to match the built up rounded spine piece.
Before I shape the extended ends I have to make the spine piece. I cut a piece of book board the width of the outer tray plus two board thicknesses to compensate for the cover boards. The height is the same as the height of the smaller tray. This rounded spine piece fits inside the extended sides of the larger tray which supports the ends of the spine when the box is closed.I cut a series of blotter strips that are cut to the same length as the spine board and are cut narrower than the piece before. They are glued up to make a rough rounded shape.
When I am going to do a lot of tooling down the spine I use book board instead of blotters so the spine is more dense.
I sand down the edges of the blotter to create a smoother rounded shape. If there are spots that don't round over well I use wood putty to fill in the voids and sand it down. After I have the shape of the spine piece, I cut the extended ends of the larger tray to match this shape. 
I then cover the tray as usual with an off white book cloth.
For the outer case of the box I cut out a piece of Stonehenge paper for the spine lining the height of the cover boards and the width of the rounded spine piece. This is what the leather will be pasted to and if I want the box to have the look of a raised cord binding I add the false bands to the paper.I used double sided tape to mount the spine piece to the rounded spine section. For this box I cut out pieces of book board to create a raised bands and glued them to the Stonehenge. The false bands were cut wider than the spine width and after being glued in place the ends are trimmed flush to the edge of the rounded shape.
The Stonehenge paper spine piece is taped in place over the rounded spine board with double sided tape so I can remove it from the spine piece after the leather is put on.When I am ready for pasting on the leather for the cover I wrap the trays in plastic wrap to protect them from the paste and put the outer cover boards in place in relationship to the trays and then it all goes into a lying press.
The leather is pasted on in two stages. First I put the leather over the spine and work it down around the false bands but do not turn over the top and bottom edges yet.
After the paste has dried and the leather is set across the spine I remove everything from the press and peel away the rounded spine board from the Stonehenge spine lining. I repaste the turn ins and set the headcaps, making sure the rounded edge of the larger tray will fit in place next to the headcap. To help in making the headcap, I use a small piece of cord using the ends to wrap the leather around. 
When the leather is dry I line the inner hinge of the cover. Here I used a orange book cloth on a different box, normally I try to match the cloth I used on the trays. I tried once to line the trays and this inner hinge with paper but the paper broke along the hinge area soon after I finished it so I always use cloth now.
The marbled papers are added to the cover and then the trays are glued in place as usual.In this photo you can see how the spine piece is the same length as the smaller tray and the rounded ends of the larger tray set down next to the headcap.
There is a video/DVD of Scott Kellar's methods for making rounded drop spine boxes at the Guild of Book Worker website. He demonstrated at the 1999 meeting in Chicago. I have used his method many times and then started adding the extended rounded tray pieces. For general direction on making drop spine boxes, also known as clamshell boxes go to:
