An apparatus and related methods for holding bags for transporting trash, or for holding displays, on the exterior of a vehicle. The apparatus includes an adjustable-length strap with a distal end and a proximal end, an engagement loop attached to the proximal end of the strap and at least one hook member attached to the distal end of the strap. The engagement loop is configured into a cow-hitch knot around a fixing means of the vehicle. To adjust the tightness of the cow-hitch knot around the fixing means, an adjustable loop-lock is provided for locking a portion of the cow-hitch knot at a desired position. The hook member consists of one or more hook-shaped, tooth-shaped or claw-shaped projections and is provided with a vacuum suction cup which is secured to a surface on the vehicle to minimize any undesired vertical or lateral movements.