When you are not using your freezer containers store them with water to save energy in your freezer. The you keep the freezer the less energy it uses. AND, you will save room in your Tupperware cupboard and the bases and seals won't get lost. Same with modular mates, but store in the cupboard empty when not in use so you will use them.
If the seals are difficult to fit on the container, simply heat them in the sun, hot water, or dishwasher, apply them to the bases right after and they will mold to fit the bases and create a lifetime liquid tight seal.