Sapper you forgot to mention that the steel bowl was placed on an engine block or exhaust to cook it with the engine running.
That would be the metal ammo box's windlassed to the exhaust pipe then. Simply chuck the compo tins in (un-opened of course) just before leaving the harbour area and dig them out on arrival.... Sadly if the convoy got lost or delayed then not all the tins made it
The tin would go off with a bang followed by a couple of bodies popping out the mortar hatch collect the surving tins before they followed...