Bald Paul does your riding partner still have the reflector bracket that is required to come with your bike and which Trek provides with every bike? If so, likely the model I have of the Madone mount might work, if the reflector mounts the same way as the Madone, it should. I don't have a Trek dealer close and there is no way they have a 2019 Domane left in stock (the pandemic has been very good for Trek since they correctly predicted demand and increased their production, while they could, and almost all their dealers have no stock left). A picture of the reflector mount might help me determine if my Madone mount might fit.