The best things in life are free, and water is kryptonite to microwave technology.
Attenuation; Water is a free attenuation solution for microwave radiation such as radar and free plastic bottles can be filled with free water, it's also a good sound attenuator.
Reflection; Aluminum is not as good for attenuation as lead sheet but it can be used for reflection and act as camouflage when radar (wall penetrating millimeter waves) or electronic warfare is involved; Aluminum rolls are relatively cheap.