Roll your own, without a doubt. Accumulate "surplus" (e.g. if you purchase a box of items but pack less than that quantity) and use it to keep consumables topped off.
The package matters greatly - it should survive immersion and extended rain. At the very least, a ziplock type of container. I prefer dry bags by OR or Sea-to-Summit as the outer package. Some folks use an appropriately sized Otter box or Pelican case... for me, those are dandy for applications other than carry (car, canoe, bike, etc.).
Some of the AMK, Atwater Carey, Phil & Jan Carey (Campmor), etc. kits are a good start - toss some items and add others. But drop the final kit or kit+ into a dry bag of some sort.
One thing I feel is often overlooked is hydration aid. After bandaids, the MOST frequently used item in all my FAKs these days is Hydralyte (formerly Gookinaid). I carry 2 1 liter packages; some trips involving a larger groups/longer duration I add more packages to the larger group aid bag. I stick to the original "un-flavor" for this use. Seems to be simplest for me to buy from REI when it's on sale.
HTH,
Tom