This paper presents a robust IIR filter design algorithm for very high Q-factor digital notch filters. The approach is based allpass substructures which allows the overall design to maintain low order while still providing linear phase. Unlike conventional allpass based filter design approaches, the proposed algorithm uses simple, intuitive trigonometry based techniques to ensure that the coefficient can be dynamically updated in changing signal. Additional consideration is given in the implementation of the structure in real time FPGA hardware. Specifically, polyphase and scatter-look-ahead methods are applied to accommodate for lower clock rate as well as timing differences between feedforward and feedback paths. The allpass substructures' symmetry is also exploited to ensure that the overall design is robust to coefficient quantization. Example results show that a 26-section notch filter design at 0.0025% of Nyquist bandwidth is able to achieve 40+ dB attenuation even after quantizing the coefficients to 16 bits.