My house has round ceiling registers for the air-conditioning system. In the winter, we’d get cold air fall-ing from the registers. Rather than put up with the drafts, I sealed the registers with those clear plastic saucers that you put under flower-pots. I temporarily glued them in place with White Lightning SEASONSeal Clear Removable Weather Stripping ($5). It’s a rubbery sealant that you apply with a caulk gun and peel off in the spring.