Seeing the containers and contents are portable rather than on the ground soil it may be simpler to do the heat treatment. I would not have hot enough outside temperature and enough direct sunlight before spring gardening season. I want to do it when the vegetables are done I have in them now.
I could purchase a metal tub like the old wash tubs and put the container mix in that depending on the size of the tub. Maybe put the tub in a three sided enclosure. Then I could put an electric space heater directing the heat to the tub and bring the container mix up to the required temperature and cook it for the length of time required . Be about the same thing as an oven without the door.
Maybe have a marshmallow roast for myself while I am tending to it.