Outdoor Recreation stores have had the Nalgene Water Bottle Kits on sale for about $20 on average. This is a good idea and inexpensive. Changes we would make is for you to buy the bottle and assemble the kit yourself and to include the purchase of a stainless steel cup (GSI makes one)that fits snugly on the bottom which can be used in a variety of ways and is inexpensive. You want to have as much versatility in the tools you buy - especially if you are concerned about the costs...we hope to help you accomplish that here by doing the experimenting for you.
One of the best reasons for DIY is that you will ultimately select better quality components to include in your kit and the process of assembling it will help you better understand how you might better utilize the kit when needed.
The concept is similar to conducting a brainstorming type training session - you may never encounter the scenario you trained for, but the experience and knowledge gained will prove invaluable in the event of an emergency.
You can carry this in your vehicle for times when you want to take a quick hike somewhere, as a component part of a larger kit or system or as the balancing 2nd "bottle" in your daypack. Have a few plastic Ziploc type bags inside to transfer the contents to when you fill the bottle with water. These won't stand out too much - even in the urban environment as Nalgene bottles are fairly commonplace. You may want to defer on using the sticker if you prefer the "gray man" concept - you can even get these with decorative exterior covers to mask the contents....just remember that most items included will rattle - not splash, so wrap or bag as appropriate.
Future articles will include a practical content lists for this type of kit.