It's been a while since you posted this (I just now saw it), but if you haven't gotten your sifter made yet you might want to look into things that are made to separate brass from cleaning media (used in reloading shooting cartridges). I have the one below and it fits on top of one of those orange 5 gallon Home Depot buckets. Works great for separating brass. Might work for cat litter too (that would depend on how fine the cat litter is vs. the holes in the sifter).Been researching sheds and planning to make my own litterbox sifter that doesn't cost $50.
https://www.amazon.com/Frankford-Arsenal-Quick-N-EZ-Separator-Reloading/dp/B004J4F09O
One of the pictures at the Amazon link above makes it look much deeper than it actually is (the other pictures are more accurate though). The sides are maybe 5" (?) high and the straining area is about one foot in diameter.
$8.49 as of now (Amazon prices can change on a heartbeat though!) Then you also have to buy the Home Depot bucket if you don't have something like that already.
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This image from an Amazon review of the sifter gives you a good idea of the size, and of the size of the sifting holes - it is placed next to everyday items that you can use as a reference to imagine the size. It's heavy duty plastic. You can shake the snot out of it and it should last a long time.
https://m.media-amazon.com/images/I/71JLRQTHAeL.jpg
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I guess if you REALLY want to know the size of the holes, this picture should tell you. You can find a picture of anything on the internet! Now you can go measure your cat's turds to see if they'll fall through (doubtful, unless your cat is very sick). The review said that .22 shell cases will fall through the holes but 9mm cases will not. So now you have a size reference for something you probably have in your pocket (I normally find these things randomly in my pockets - I assume everybody does!)
https://m.media-amazon.com/images/I/61h2+st3SSL.jpg
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