I understand this probably won't pass muster for a lot of people, but the easiest thing I've found is to buy some 16 oz. reusable translucent round food containers with lids.
I fill these with a cup and a quarter of my chosen cereal, put the lid on and do a week at a time or more. That leaves me room for milk, oat milk, almond milk or whatever.
I do this because I get cheap Walmart Great Value Bran Flakes that have NO raisins, lots of bran, and lots of fiber without so much sugar as other brands with the hated (by me) raisins. WM bran flakes have 120 calories/cup; 1 gm fat; 7gm fiber; 6 gm sugar.
Compare to Kellogs Raisin Bran: 190 calories/cup; 1 gm fat; 7 gm fiber; and 17 gm sugar!
Anyway, for those maybe how would be interested it is the easiest thing I've found!