That looks pretty nice and simple. I do have a few suggestions though.
Are the C-clamps all that is holding it on? I would have some concern that the vibration of the motor might jiggle those loose. One suggestion I have is to loop the trolling motor wires once around that rear seat when you are using it. Then even if it did work loose and fall off you would not lose the motor. Another idea would be to add an eye bolt to the board and chain that to the seat. I would also drill/carve some shallow countersink holes for the C-clamps so that they sink into the 2x4.
Another simple suggestion I would be to widen the top where you go over the gunnels by gluing extra a piece of 2x4 on the front and the back. A trolling motor will create a lot of torque on that 2x4 and tripling the width of the base around which it will want to rotate will help keep the transom from rotating.