I’m looking at my Amazon Affiliates sales report and saw that somebody bought one of these.
This “Mule Surf and SUP Paddle Board Carrier Transport System” allows you to tow a long board or SUP with your bicycles. Straps secure the two wheels to the board, converting the board into a trailer. Another set of straps wrap around the saddle.
The setups for long boards I’ve seen in Santa Cruz involve some kind of chassis that the board rests on. I have no experience with any of these carrier systems for SUPs or long boards; caveat emptor.