Linkstar has a wide assortment softboxes which can be mounted with an adapter ring (Speed ring) on various brands of flash units. This way you can save a considerable amount on original softboxes from other brands. The outer diameter is 15.4 cm.
The Linkstar softboxes come standard with a speed ring, which can be replaced by this ring. The ring is placed in its entirety around the front of the flash unit. This adapter ring is suitable for your Profoto flash unit.