Many thanks Mauriz. What you suggest works in my opinion; it seems feasible given what we can see of the inner frame.
Wouldn't it be interesting to see the tiara and examine the metal work inside the jewel.
I find the wording in the Garrard's ledger confusing but I suspect we need to take into account that the entry was written by a clerk, not a historian of jewellery.
Does Munn comment on how the tiara was constructed?