if you use the Jello mold oven here's a shot of the real thing. i got this as a gift at the stove collectors camp out,it's old and made in Sweden.the instructions in several languages suggest going into the galley to check the baking progress so i assume it was made for boating and not camping.
my guess a diffuser. maybe even put a little water in it and the set the JM on top. that way it gets some heat from the bottom, but most is coming from the top via the cone and domed lid. just like a dutch oven is supposed to be heated. most of it from the top.
it's more of a heat sink than a deflector.made of steel rather than aluminum it keeps the cooking pan from overheating,it also provides a place for the oven to sit on the pan rests of a Primus stove.there are a few small holes on the side the blue cover.i assume the Optimus people worked out just the right hot air flow thru this.at some point this summer i'll give it a try it.the instructions say to run the stove at max heat for seven minutes then reduce to moderate heat and it will cook like a conventional oven.it came with recipes for rye bread and ham au gratin and some Swedish dish called jansson's temptation which involves fish and potatoes---