I'm going to go with a case of air horns from the dollar store for now. Those things are LOUD!!!!!!
that's true but just think about how loud it will be in that shelter, but with it being registered and friends and family knowing where to look you should be ok.
Eck, when we buy our bottled water the new case goes to the shelter and the case from the shelter goes to the house. my shelter is concrete and yes my cell phone works from inside.