The term 'boot fillers' refers to items that are placed inside boots to make them fit better and provide extra cushioning. Common boot fillers include insoles, heel lifts, and arch supports. These items are designed to provide comfort and support for the FEET while wearing boots. The crossword solution for the term 'boot fillers' is 'FEET', which is an appropriate answer since the items are designed to provide comfort and support for the FEET.