The transom assembly that I previously built was mounted and braced onto the rear of the shelves, and then set with stainless screws and epoxy. I found a tight-grained and dense piece of Douglas Fir for the motor mount area, but next time (?) I would use a section of 1.75” LVL to strengthen this area to accommodate larger outboards, such as a 150 hp Honda.

 

Note: for long straight edges, I use a variety of aluminum angles in lengths up to 8 feet. These are great for setting levels on to check across the width of the hull and to use as saw guides for cutting plywood.