Modular feet do work.
This is a simple foot the same size as the original Otto feet, but with a hint of toes at one end.
I only have an overlapping area at the ends here, and it might be a good idea do have a slight overlap along the long edge too as it would help hide the very small, but still visible, gap. Alternatively, having a robot above it will also hide it most of the time.
There also needs to be a very small (0.5mm?) chamfer on the vertical edges at the corners of the ankle. The square corners of the toes cutaway section rounded slightly as they printed, and so there or the ankle corners needed trimming afterwards.
The toes were printed up-side-down to avoid having the overlap as a supported area. Obviously this limits the design of the toes. It might be possible to print that piece the right way up if a single wall support is added near the edge of the overlap.
I used M1.4 3mm screws, with 1.5 millimeter clearance holes in the ankle and 1mm grip holes in the foot. The foot holes worked well, but the clearance holes needed drilling out - they did not let the screws through cleanly. These were pan head screws, and a 3mm wide 1mm deep recess in the bottom of the ankle was enough to hold the heads. With four screws the joint seems pretty robust, and will withstand considerable force being used to twist, turn and pull it apart. If I tried harder I am not sure whether the screws would rip out of the plastic or the plastic would snap first.