You need to make sure your displacement tool won’t float to the top, so you’ll need to fill it with something that weighs the same or more than water.
Sand and water probably come to mind. Well, since sand could still potentially cause some harm to your plumbing, and because water is readily available, water is probably your best (and easiest) bet. Fill your container a half an inch to an inch from the top and screw the lid on tightly.
