I wouldn't reuse baggies because I can't see them being airtight after use and washing. They're designed to be thrown away, and as such, they are very thin. This is kind of the same "problem" caused by bottled water manufacturers recently "going green"; they make their bottles thinner now to "reduce landfill space usage", but ironically that makes it harder to reuse them.
If your water heater is sufficiently insulated, a timer isn't going to save any money because it will not have cooled down significantly by the next time someone demands hot water. More effective solutions include insulating the hot water lines in the basement, reducing the water temp to 130 degrees F, wrapping the tank or replacing it with a more insulated tank*, and switching to an "on demand" system.
* Who the heck has all these uninsulated tanks? Mine is 20 years old, and it feels just barely warm only on the top.