A symmetric pair of mounting brackets for mounting one of a plurality of air filters to a mounting plate of an air conditioner unit are provided. The mounting brackets each define a plurality of mounting surfaces with equidistantly space mounting holes. The mounting surfaces are spaced apart from a plurality of retention surfaces, with the distance between each respective mounting surface and retention surface varying to accommodate a different filter size such that the air conditioner may operate using different filter sizes by simply changing the installation orientation of the mounting brackets.