Stephen Stewart had the box for his Mog campervan made by (IIRC) a reefer box company.
http://www.xor.org.uk/unimog/index.html
The walls were 30mm and he said that if he had to do it again, he would have gone with 60mm instead - at least around the sleeping berth.
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Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker