I recently purchased a 2014 Hobie Outback Kayak for fishing. It came with the “Lowrance Ready” transducer mounting plate on the bottom, which is great, if you use Lowrance fishfinders and GPS. I don’t. I’m a Garmin guy, so I needed to figure out how to mount my Striker 4’s CV transducer in this area. There are people who make custom mounting plates using 3-D printing with carbon fiber, and these are between $25-$30. I am sure they are nice, but I came up with a solution for less than a quarter using an old AC outlet box that I could cut up. Any similarly sized piece of plastic (or metal) would also work, I just used what I had.

Using a Dremel, I cut out 2 rectangular pieces of plastic
Place the plastic strips along each side of the transducer mount and align with front edge of Hobie mounting uprights. Drill hole for bolts through the existing Garmin mount hole and between the Hobie plate uprights.
You need to fill the area between the Hobie upright pieces with something Anything would work, I chose a bunch of stainless washers that I already had in my pile.
You want the transducer to “float” just a little about the Hobie mounting plate, not sitting on it. I put a thin piece of cardboard between them. I used the Hobie metal clips that go up against the uprights, between the bolt head on one side and the nut on the other. This adds a lot of strength to this area and prevents the small upright plastic parts from separating. Tighten all the bolts.
Final cleanup. Cut bolts to correct length, trim excess plastic, etc. It doesn’t have to be pretty, nobody will ever see it again.
These big transducers will not fit completely under the Hobie plate. They will stick out about an inch, but will not do any harm. Notice in this picture that the mounting plate does not touch the kayak at the rear. This is a problem.
It is possible to bend the Hobie plate down and screw it in. This puts a lot of pressure on the Hobie mounting plate and transducer. I did this only for a test and did not like the results, so I took the rear screws out.
The final solution was a short stack of 8 stainless washers under the Hobie mounting plate on each of the rear screws. I had to buy longer screws than the Hobie ones, about 1″, and then shortened them a bit before installing them. These screws had smaller heads than the Hobie screws, so I put a washer under each of them.