Using a home insulation product, line the interior of the box using a spray-on glue and a filet of silicone at the corners. This does wonders for rigidity without adding unduly to the weight (and even those wings carrying people into space nowadays are skinned foam fabrications). In reality at full scale this won't be a closed compartment because access has still to be accommodated for ingress by stepping back into the booth from out front.
Note the orientation arrow is duplicated up top prior to its being covered at the base!
