I always find it suspicious when a manufacturer doesn’t specify what “foam” they use in their products. That’s a deal breaker. XPS or bust.
EPS and Polyurethane CAN work, as long as the box is properly reinforced, so the load is taken off the core material of the sandwich. If you REALLY want a structural sandwich panel, you will need to use something like Gurit or Divinycell and a carbon, fiberglasss or Aramid skin, impregnated with epoxy or vinylester resin.
I built a 12' x 7' x 7' box with a friend using thin phenolic sheeting, 2" EPS, wood nailing/screwing blocks and 16 gauge steel angle iron as a perimeter frame. That box was RIGIDLY mounted to an LMTV and the truck was driven from Alaska to Baja without any issues.
Four point mounting systems, vacuum infusion and $50,000 of Nomex honeycomb are not required. You can build a box or a trailer with Vetroresina or G9, angle iron, industrial spray glue and XPS sheeting from the DIY store. Overlap foam and sheeting to make larger panels, using 4x8 or 2x8 sheets.
DO NOT use polyisocyanurate foam unless you have an external and/or internal support structure. The foam is a crumbly mess.
Commercial refrigeration panels are also an option and are often constructed using stainless or aluminum. Exterior and interior can be sprayed with aliphatic polyurea to seal everything up.
