When you build the second version, you may wish to consider composite panels. There is no nasty fiberglass to deal with, no chemicals to mix, no sanding, brushing, etc.,
A customer of mine has panels to make a 12 foot long by 7 foot high by 7 foot wide RV box and the panels all together weigh well under 500 pounds. The walls are about two inches thick and they have an R value of of about R-30 with a special heat reflective coating sprayed on the outside after final assembly. The panels are bonded together with a something along the lines of Sika 252, depending on the composition of your panel. Thin aluminum or composite extrusions can be used on the exterior to further seal the joints and offer some protection against branches and such. It should look something like this when completed:
so if that box empty weighs about #500, the one for your JKU should not be anything near that, which is what you have now.