Ok, having fitted my chimney hood I've ended up with a small gap where the vertical channel and the ceiling are supposed to meet flush - the gap is around 3 to 4 mm all the way around, not massive but enough to make it look untidy and you can just see the ducting pipe as it enters the ceiling. Couldn't get the thing any closer. Any advice on what I should fill the gap with? Would silicone sealer be ok? It's brushed stainless material.