I lost the annex to my van. We believe it blew off roof of the car about a week ago while driving between the Jarrahdene camp (on Caves Road), which is near Margaret River, and Augusta. The annex was in a bag that is silver/grey in colour and is about 1m x 800mm wide. We've reported it to the police, posted on the AugustaMargaretRiver Facebook community bulletin board and put up posters in the community bulletin board but have had no response.
Make sure to keep checking Gumtree sale adverts. I think you can also post a Lost adver 0n Gumtree.
Both of the area local newspapers The Augusta-Margaret River Mail and the Busselton-Margaret River Times offer free Lost classified advertising. I think you can contact them via email or phone to accomplish that.
Consider emailing or faxing your poster (does it include a photo of the annex set up?) to each of the caravan parks in the Cape to Cape area - there aren't that many. Ask them to put it up where people will see it when they check in and out. Offer a reward.
You may want to do the same with the few backpacker hostels. And the tourist bureau.
Have you checked along Caves Rd if still in the area? Things can end up quite a bit off the road way!