Rather than make something, I elected to install ready-made products to get my fishfinder and transducer mounted on my I9s.
First, I installed a Hobie Accessory Holder Kit ($21). This gets glued on.
Next I mounted a "Liberator" ($39.99 from kayakfishinggear.com) to the accessory holder. This consists of a 10" x 4.5" platform and a folding transducer deployment arm. Mounting hardware was a bit of a hassle. The screws you need to mount anything to the Hobie accessory holder is 1/4x20, which is the most common machine screw on the planet. However, length is critical. Too long, and the screw ends up being driven into your kayak. 1" was a bit too long and 3/4" was too short. I ended up counter sinking the mounting holes (with a spade-type bit) so that the 3/4" screws would reach. If you do this, counter sink first, then drill the mounting hole! The material that the Liberator is made of is very easy to work with. Kinda like what a cutting board is made of (maybe a little softer)! Also notice the Liberator is not a rectangle, but has a "wing" up front to put the transducer arm where it needs to be.
For power, I found a 10 AA battery holder ($2.30 from thomasdistributing.com) for my 2700 mHh rechargeable batteries that I found at Fryes. This provides a full 12 volts and I expect plenty of run time.
I mounted a box with velcro to store all the cables and the battery pack. The transducer deployment arm folds up and can stay on the kayak when the kayak is folded.
View from inside the kayak:
View from outside the kayak:
Kayak ready to fold:
10 AA Battery holder w/ 9V-style snap connector: