I uploaded in Thingiverse an extension of the board holder, which encapsulates the daisy extension board and also has room to accommodate an 1100 mAh Li-Po battery, which may be changed avoiding unscrewing the main board. The opening in the bottom allows changing the battery, which connects to the main board through a JST-PH socket accessible by the exterior of the box.
There are two main board holders, one for the Mark III Supernova version and another for the new Mark IV (the fixations are different).
After printing the parts, a JST-PH extension cable (like this one, but not so long https://www.adafruit.com/product/1131) should be fixed (e.g. using hot melt). The plug connects to the back of the main board. The cable should pass by the left of the pin in the spring, so the battery does not entangle. A strap of electric or masking tape may be fastened to the battery in order to improve friction.