After getting the exterior trim in place, I was able to measure the dimensions for the 'roof' of the tower. I bought a 2'x2'x3/4" piece of plywood for the top and cut it to the size of the outside dimensions of the trim. Then I glued and nailed 3/4"x1 1/2" solid wood strips around the outside edges to prevent weathering. I centered the insulated top on the roof, marked and cut an opening to match the opening in the top and sun collar. Then I screwed those three pieces together.