Speaking of Thru-Axle Rack Mounts: Robert Axle Project's Cargo Thru Axle | The Radavist | A group of individuals who share a love of cycling and the outdoors.
Specialized Rear Rack Seat Collar - Components
Amazon.com: Vincita Rack Mount Seatpost Clamp - For Bike Without Rear Rack Threaded Hole Fixation - Suitable With Seatpost Diameter 31.8 mm (Seatpost Tube 34.9 mm) - Lightweight -mountain, Road, Folding Bicycle. : Sports & Outdoors