Outside of trawling Aliexpress or a custom metal shop, you could see if there are any magnetic step up rings for lens filters that would work. I see some in the 105mm range but no idea if the polarity would be right.
is it just a flat plate? you could get one laser cut https://www.superbestaudiofriends.org/index.php?threads/resources-for-diyers.10300/#post-329087
something like this perhaps: https://www.amazon.com/105mm-Stainless-Steel-Internal-Retaining/dp/B07YN4MND2
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