I want to directly replace an existing steel tube structure (the vertical fin on an experimental Piper) with a foam glass part because I think it's a fine place to hide a comm antenna. Most structures like this, including the H/S, rudder, etc, are attached with a simple tube-into-tube connection secured with a bolt or two. Any luck doing this with foam/glass? If you don't want to change the aircraft structure, how do you create the tube interface in the composite part?