A device for adapting a cover for an existing air filter assembly to seal an end portion of a replacement cylindrical air filter having a round cross-sectional shape is disclosed. The cover has a plate with a radially inner convex guide separated from a radially outer convex guide by a concave receiver, a tightening member connected to the plate, and a catch having a first end movably connected to the tightening member, and the catch having a second end configured to engage a keeper, the tightening member adjustable to vary a distance between the catch and the plate. An adaptor is attachable to the plate of the cover. The adaptor has a first side for engagement with the plate and an opposing second side for engagement with an end portion of a cylindrical filter. The first side has a sealing surface extending radially inwardly from a peripheral edge. The sealing surface has a sufficient area to overlap a portion of the radially inner guide of the plate and a portion of the receiver when the adaptor is attached to the plate.