I've made a stern plate out of ABS so that I can mount my transducer without marring my transom, like my mast mount on the port side. I've been experimenting with transducer height as shown here, but I'm not pleased with the outcome. Can't get a position which allows me to scan on plane.
This is the setup I made for my old boat about 12 years ago. It has served me well.
The boat was rope start with no battery. The mount is made from ABS plumbing pipe like that shown on the right of the first pic.
There's ample room underneath to store excess cables and accessories, etc.