For a down locking threaded barrel seat I'd suggest a Struble either D1, D3, D8 or D20. Anyone of them are good hardware so it's just a choice of which one looks best to you. About the best price on them comes from J.Stockard http://www.jsflyfishing.com/cgi-bin/category/43700 and you can get just the hardware if you click the drop down menu for the insert choice there is a choice that says "skeleton only". That way you can turn your own wood insert if you like.

Steve