That got me thinking about a "bag" that was smaller than an old sleeping bag stuff sack. Along came the fast bottle!
Step 1: Shrinking the "bag"
Then I remembered that lots of people were using the old BPA water bottles for survival kits, dry storage, etc.
I raided the cupboard and found a bike bottle of questionable plastic content (even though I REALLY love riding for the NCMEC every year)!