James, one way with added benefits, is to use the 3/8 and then holow out the center, put ribs in it then skin it with light 1/16. It will be lighter and stronger. This is how I build all my tail surfaces. sounds like the stab is built the same way so just carry on. Dont forget to leave enough meat in the 3/8 to hold the control horn wire though, its a bit tough to do it lol