Mostly harmless has the correct - shelter is missing here (and in most peoples kits). A disposable poncho and a space blanket would be cheap, compact and handy (even with their limitations they would be 100% better than nothing). I like ponchos as they can be useful even if you aren't bivvying up.

Food also useful - even a couple of hard candies would give a little boost when needed.

Also a plastic bag and puritabs

Your vision of what you want to do is fine - I would drop the body powder to add some of these things