For a full body design how about:

1) Getting rid of the slider and make the screen and keyboard in the same plane. Than the device will be wider, and you can also fit two normal batteries side by side 90 degrees turned. In this case the device can be both thinner and have bigger battery capacity. Maybe the screen can be even a few degrees tilted, in a fixed position. The device will not be so flat and fit better in pants with curvature of leg.

2) Or else on the same plane as the current battery is in, have two batteries side by side. Then device will have same thicness as current device but bigger battary capacity. But the body will be a little wider by 3 or 4 mm.

Edit: I would definately be interested in whatever you cook up