The sides of the motor chamber are basically extended to form the sides of the flare. In most cases, this common side is made from a single sheet of plywood. As such, the motor chamber is attached to the flare. We've made some versions I called a "road horn" where the horn came apart and was held together with clamps. Each section was gasketed with compressible rubber. In this case, the flare and motor chamber are separate but the basic layout is the same.