However, really think I would have detected a beneficial ground effect with the air gaps in my last experiment.
In fact I find this quite strange. If I find a bit of time I will try again with a 84 mm top with flat bottom like the your, to see what it happens.
AMI (I3): 8.2e-4 kg m²
This is 0.00082 kg m2 ?
It seems a very high value, my copper top Nr. 22 weighs 656 grams, weight concentrated outwards, diameter 80 mm, and the AMI is 0.00064, less than this your 165 grams/84 mm top. It doesn't seem possible.