All the kits come with a bunch of spares (arm, centreplate, stackup plate, and leg set) We decided to use one of the spare leg sets that are included as a sonar mount. It is reccomended that you mount the sonar at least 3 inches away from the body to avoid electrical noise/interferance, the setup about is just over 3 inches away, but If you want to mount it further out, you can do so but turning the leg over.
You will need to buy some extra screws and nuts (M3 x 25mm). I would suggest also getting some double sided tape to secure the sonar, as there is only a single hole to mount it on the end of the leg.
Hope this is usefull for anyone who has not figured this out yet.