Abstract: A temperature-controlled fluid processing system with accessory attachment mechanism includes an accessory annular ring feature a mounting surface and a tapered surface sloping toward a central axis of the ring feature and away from the mounting surface, and engaging components on the primary device, including concentric annular member and rotation plate. The annular member includes spaced apart through holes housing spring biased pins. The rotation plate surrounds the annular member, and includes one or more ramped inner surface(s). The plate is rotatable between an unattached position wherein the ramped surfaces apply no radial inward force to the spring-biased pins, and an attached position wherein each ramped surface displaces a pin inward through its corresponding through hole such that the conical tip of the spring-biased pin engages the tapered surface of the ring feature, thereby immobilizing the accessory with respect to a head unit of the fluid processing device.