Do a search for tube to tube carbon frame building. Tubes are cut, mitered, tacked with appropriate glue, and then the joints are overlaid with more carbon; either wet layup or pre-preg to acheive the strength needed for the structure. Many custom carbon builders use this method, ( Crumpton, Calfee, Appleman, ....) it allows the freedom to design without the constraints and tooling costs for molds.