Introduction. This article details the development and use of a low-cost, custom RMST device for a patient with long COVID who had received positive airway flow support during ICU treatment. By sharing our successful management of respiratory muscle weakness in a severe COVID-19 patient, we aim to contribute to the broader conversation around effective long COVID management. Case Description. A patient with respiratory muscle weakness used a hospital-made RMST device. The training involved ten short forced exhalations per cycle for five cycles, followed by five long forced exhalations held for five seconds. Additionally, the patient learned lower abdominal and box breathing techniques. The patient showed significant improvement, using the RMST device without supplemental oxygen within 24 hours and completely weaned off by day 5. By discharge, the patient could complete the RMST exercise regime and achieved 290 meters in the 6MWT. After six weeks of outpatient therapy, the patient used the RMST device at 80 cm H2O and increased their 6MWT distance to 390 meters. Device Description. The RMST device was designed for in-house production with a 3D-printed PETG cap, base, and plunger, and a standard pen spring. Its design mimicked a standard PEEP valve with a different geometry. The spring closed the valve until a pressure threshold was reached, allowing airflow. The adjustable pressure threshold ranged from −40 to +40 cm H2O, calibrated in 10 cm H2O increments by measuring displaced water column height. Discussion and Limitation. COVID-19 can cause long-term respiratory issues needing proper management and rehabilitation. Inspiratory muscle training benefits those recovering from severe COVID-19 who were weaned from mechanical ventilation. However, the device’s design and production method are unsuitable for large-scale and commercial production. Rehabilitation centers should prepare for postintensive care syndrome in post-COVID-19 individuals, with interprofessional teams addressing various recovery aspects. Early medical attention and therapy can improve patients’ quality of life.