A composite back is a good idea. It would allow for some subtle curves and soft rubber inlays to make it easier to hold the tablet with one hand.
Carbon is cheap and very easy to work with these days, so it should be cheaper to produce than the aluminium back. You could make it even cheaper by using glass instead of carbon. You wouldn't get that black weave look, but you could get some interesting effects using pigmented epoxies.
I'm tempted to have a play around with this myself. Once you have the moulds set up it should be simple to create a range of different custom looks using different materials.