The rectangular box-shape of the body was constructed from three separate pieces. The front surface and side surfaces were made from a single piece of cardboard with two small flanges - one on each side - for easy attachment of the back surface. We attached the pieces together using liquid nails. Sheet metal screws were used to cinch the pieces together and hold them in place while the glue cured. This provides the added benefit of allowing the partially assembled box to be handled before the glue is dry.
Next the end cap was added. This too had flanges that were used for gluing to the body section. A square shaped hole was later cut into this end cap to allow my son's head to fit through.
Arm holes then need to be added on the side surfaces. Make these oversized to facilitate the process of putting the costume on and taking it off.
Tip: score the cardboard before bending to allow for much cleaner folds.