An adapter is provided for placement within a furnace filter bay having a flow of air therethrough. The filter bay is for receiving a furnace filter. The adapter includes a filter abutment face for abutment with at least a portion of the furnace filter, a mounting face parallel to the flow of air and attachment means permitting removable attachment of the adapter to the furnace filter bay. The adapter allows filters of smaller depth than the filters originally designed for use within the filter bay to be used, thereby eliminating the need for purchasing the original filters. A brace is also provided for the same purpose. A kit is described that includes the adapter and/or brace, along with instructions for installation thereof within the filter bay.