Additive manufacturing enables the fabrication of geometries previously unavailable via conventional means. Here, a helically coiled channel containing a secondary helical coil insert was 3D-printed and operated in a net flow with superimposed oscillation. This "coil-in-coil" geometry exhibited exceptional plug flow quality at low Reynolds numbers (Re = 10-50). High levels of plug flow were observed at oscillation amplitudes of 2-8 mm. The greatest degree of plug flow occurred with Re-o/Re-n = 2-7 and Sr = 1-2. Surprisingly, the coil-in-coil geometry demonstrated excellent plug flow behavior (N = 40-100) at very low oscillation frequencies (f = 0.25-0.5 Hz). Compared to various benchmark designs ("plain" coiled tube, straight tube containing a helical coil insert, and plain straight tube), the coil-in-coil geometry demonstrated enhanced plug flow quality at low frequencies (<= 1 Hz) at all amplitudes and low amplitudes (1-2 mm) at all frequencies.