Any tips for making little supplement packets?

Do not use film cannisters or pill bottles!!!! Film canisters have a ton of gnarly chemicals in them and pill bottles could have residue from medications. Why someone would purchase things to make their horse healthy and then store it in a chemical container is beyond me. Totally counter-productive.

I do the plastic bag method. Works fine for me.

SmartPak is also good, but a tad expensive, they will ship the products to you all ready measured, in little tubs.

You could use old film cannisters, pill bottles. It depends how big thy need to be. Maybe if it's just a very small amount you could use one of those plastic pill boxes with the little sections for day of the week and time of day? They seal quite well. Are you using pea protein as a supplement? I highly recommend it as a source of vegan and gluten free protein.