There are many ways to build a box, I built mine from a gfk sandwich with 5 mm foam core and 3d Printed inserts from ASA-X plus top and bottom plate from a cfk sandwich. See here: Daniels (un)geared inflatable (slowly built)
It still works with some modifications. The bottom plate could be re-done in one piece of cfk sandwich without the aluminium plate, that would further reduce the weight.