I found the perfect size box for my purposes here-
Closer to Wholesale, They also have
an ebay store.I got the 42.5"L x 20.5"H x 24.75"W model. Sorry, no pics of it on the truck.
Added some removable casters and a hitch mount from Tractor Supply
An exhaust opening and some ventilation, which I discovered I need a LOT more of.
The eyehooks in the lower corners are for a turnbuckle that goes between the truck's tow hooks and the the gen box. After snugging them down the box is firmly attached to the truck and doesn't wobble around like a lot of hitch mounted cargo carriers.
