In this case the trunk garment is the device to give treatment to the lymphatic system located in the human's body trunk. All the garments provided with the Flexitouch Plus equipment are part of "tecnología vestible", "electrónica textil", "tecnología corporal", "prendas inteligentes". So "la prenda para el tronco" is ok, provided there is a previous explanation these garments are the devices to give the ultimate treatment, being in contact with the patient's body. Also the other garments are "prendas" for other body parts. I didn't find a better word for it. Maybe "prenda de tratamiento para el tronco, prenda terapéutica para el tronco", etc. would work for you.