The arms of the structure must be added to the main pipes but it would be very weak by itself, so I had to strengthen it with some ribs between the arms.
The ribs are plastic sheets shaped in the extremes with a curve to fit them between the pipes and glue them.
Structure of the water wheel:
The last step is to close the structure glueing lids in both extremes of the main pipe. One of the lids has a hole where I can put a hosepipe inside to control the water flow inside the structure.