I am working on an analog notch filter design to add. It will also utilize arrays of resistors as 1% and 5% resistor arrays are available. Selecting capacitors for size will be more complicated. The current schematic on github as "notch" uses arrays and an 0402 version of the Twin T notch filter is coming. I take feedback from the same op-amp and the Sallen Key is now 3rd order and much stronger. All this for only an additional capacitor per input over a non-active notch design added to the current ECG buffer design. I'll be working on the board layout to help improve my own testing of the ECG sensors.