I have mounted mine both for testing and just used silicone glue and it held fine.
One of mine is a big 8 degree transducer.
Go to Cabela's and type in mounting plate and it will come up on the first page.
Click on it and it will show both vertical and horizontal mounting plates.
The advantage to these plates is if you buy ten diferent tranducer over the
life of your boat you still only have to make two holes.
I belive any plate you can screw your transducer to bigger
than 4 by 4 inches will hold even with silicone seal.
Want to be sure then use 5200 or 4200.
Mine I use silicone seal and mounted so I could use one of the screws from the pitot tube to hold one side.
Has been working fine for over 20 years on a Aluminum hull.
With Silcone seal you can remove with out too much effort.
If you use 5200 or 4200 will take more work to remove.
Anyway you mount it you should put a cable tie or something to hold the cable close to the transducer.
This is just it case you hit a log or something that rips it off the transom.
Without the cable tie near the transducer the transducer would fly off the transom,
and swing on the cable up and around into the boat and likely hit someone in the back of the boat.
Below is a link to the mounting plate here at IBoats.
Had to search 171 pages to find, there search is not very good.
You see most of the catalog before you find your match.
http://www.iboats.com/mall/index.cg...category=f&page_number=39&search_type=keyword
