This is a tough one and a lot depends on your time, budget and plans. A lot of builds use either a steel or aluminum frame and bond composite panels to the frame. This makes a strong box but a bit heavy. Another option is to go full composite, which has a couple of advantages at the expense of not being quite as strong. The advantages are no thermal bridging and a lighter weight. We ended up using a box from Total Composites. They supply custom made panels with the extrusions and adhesives which greatly reduce the time to build in comparison to other methods. Now after living in the habitat I can say that it is extremely well insulated. We also decided to use composite honeycomb cored panels (also from Total Composites) for the main bulkheads. I did this to have the interior walls match the exterior walls and provide a strong support for the roof. I a really happy with this decision and recommend it to anyone undertaking a project like this.