Hi Tom, Welcome to the site. I believe that the inside of the box is probably built similar to my wells cargo trailer. It has aluminum sheet siding screwed directly to the metal framing. the framing is a "hat" section that leaves a void that you want to insulate. then there are wooden rub rails horizontal on the inside. I stripped of the interior wood and glued in sheet insulation between the vertical studs sealing the insulation to studs with spray foam in a can. for the inner stud cavity I drilled a small hole near the top and used a piece of flexible tubing attached to the foam spray can I slid it down to the bottom of the cavity and slowly withdrew it as the foam flowed into the cavity. If you try just squirting foam in it will only go a couple of inches before expanding and creating a blockage. The foam comes in two types, one is a high expansion type and the other a low expansion type for use around windows. Make sure you use a low expansion foam or else you will bow the siding out due to expansion. Its a real pain to do, but I ended up with a well insulated trailer that stays cool in summer and dry with no condensation in the winter. (I keep motorcycles and riding gear in it year round.) Good luck with your project.
Dave
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