A frame assembly for releasably maintaining an air filter. In some embodiments, the frame assembly includes a thermoformed body integrally forming a base, opposing side walls, opposing end walls, and at least one closure sub-assembly. The frame assembly is configured to releasable retain an air filter relative to the base via the closure sub-assembly. In other embodiments, the frame assembly defines, in a flat state, foldable side and end panels at the perimeter edges of a central grid panel; the frame assembly can be transitioned to a final state for receiving an air filter by folding the panels. The frame assemblies of the present disclosure are re-usable, allowing a user to replace a “used” air filter with a “new” air filter.