  We propose to use an elongated rectangular waveguide near its cutoff
frequency for axionic dark matter searches. The detector's large surface area
allows for significant signal power, while its narrow transverse dimension and
tapered-waveguide coupling suppress parasitic modes. The proposed system can
fit inside a solenoid magnet and is sensitive to the QCD-axion in the axion
mass $40-400\,\mu$eV. We describe the theoretical principles of the new design,
present simulation results, and discuss the implementation.
