I don't know your situation or what you need, but I would have expected   you really only need something for initial setting only...and everything after that is  a  bit wider, or a bit narrower.

With that in mind  I would  just have  a piece of  rod  bent  to make it convenient  to  slip between the  louvres,    In my case  that  rod would be 3/8 inch one end, and  3/4 inch the other.    The dimensions you need  may well be different, depending on your sieves, and what crops you grow.

  The sundial  type of adjuster out the side would I expect be good, but it likely is a good idea  to make yourself have a good look at your sieves when adjusting anyway.

I know better than to suggest you make  the  tool out of metric rod, even if that's what I really think.....     :  )