RE: Craft Air Cowboy Build
Not to belabor an issue but Poplar is "light-ply". You should be able to order it through a hobby shop or through a place like Balsa USA.
Or "light planes fly better" so another option would be to use balsa with 1/16" birch ply "doublers" in the engine, fuel tank, wing saddle area. Modify the fuse formers for any changes in fuse side thickness.
Whatever you decide, good luck with your build.
Mike