Could you post a picture of the gap, just so see how it looks.
I would ask FB if they could provide you with a refractory material to fill the gap. It could help some.
Not sure what the others think, I don't know FB products very well, but to have your dome dropping in temp so fast, I am thinking more and more that the insulation of the oven is not proper.
Unless the dome is very thin and therefore has very little thermal mass.
Do you know how thick is your dome?
May be someone else with an oven similar to yours can share how quickly their oven temp drops.