Hi! The piece of wood shown in the pic was all I had at the moment so that’s why it’s so thick. The one I plan to make will be much thinner. The link to the original post about the “Seeley Size” I mentioned is here: Is this an active robbery? - #4 by JeffH
Foxhound Bee Company sells an entrance reducer based on these dimensions but I don’t think it will fit in the flow hive entrance.
Hope this helps!