If you buy the 12UNF female to male center adapter, then you can make the plate yourself with a piece of aluminium.
If you have 12UNF tap and die, you can make the center adapter yourself as well.
Or you can pay a machine shop to have the whole thing done.
You can carve a ring on the adapter plate and use a matching O ring or simply use a gasket or silicone sealant for the surface seal.