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I would not add grit to the peck block, offer that free choice, the birds will pick up what they need. You can place grit in a container attached to a post of your run or just scatter it around.
As for your ingredients and binding, you may find some ideas in the recipe links below.
Remember that peck blocks would generally be considered as a treat, so you do want to limit how much your chickens get at a time.
https://the-chicken-chick.com/flock-block-substitute-recipe-healthy/
https://www.theprairiehomestead.com/2013/10/diy-flock-block.html