Please note this is a simplistic design just to make it easy to build. Two things would undoubtedly improve performance.
Firstly:
The flat windblade plates would be better as a suitable aerofoil section.

Secondly:
For best performance,  there would be twist in the windblades to ensure that the angle of attack was more constant along their length.

However, even in the version as shown, tests show a remarkably good performance so there is potential for further improvement.

Do not be discouraged from making it. You will be surprised at how well it works using the large blue colour propeller as shown in the picture.