non-jamming/clogging cesspool mixer

My grandfather has a holiday-house which is not connected to the sewage system. It therefore has a cesspool with a mixer inside that turns once or twice a day for some time at 1000 rpm in both directions. The original mixer came off and sunk to the bottom, so the best option was to print him a new one. It had to:
- fit through a hole of 12.5cm (