Here are some pannier tanks made from 10L spring water tanks. The hooks are made from the steel panels that cover the card slots at the back of a desktop computer case (some of these are a heavier guage than others). The hooks fit tightly onto the pannier rack so that it doesn’t rattle or bounce off. An octopus strap or an old inner tube can tie it even more securely to the rack. The hooks are pop riveted onto the tank with some washers on the inside. They easily carried 4kgs of soy milk.