Alastair Robinson (robinsonb5 here at Eab) did work on a adapter and made the sources public:
[ Show youtube player ] Info:
http://retroramblings.net/?p=912 +
https://github.com/robinsonb5/SegaToCD32 . 8 years since last update but a lot of info to get from those links!
Also this thread is good:
https://eab.abime.net/showthread.php?t=22633
Looks like these adapters featured in the first post have used the same board, size of adapter seems to match this project