I built one in 1979 for a couple canoes up at my folks cabin. It was 1.25" thick ash with a 1.5" thick, slightly angled motor block. I bought the big bronze bolts and tail nuts from Old Town. The motor block was held to the crossbar with stainless carriage bolts and it was finished with numerous coats of Deks Olje #1. I installed a small curved thwart behind the stern seat with two bolt holes in it on one boat. The other had a second, loose curved wooden bar that fit under the gunwales, sandwiching them to hold the mount in place. Unfortunately, somebody left it out under the porch for a few years, so it's rather weathered, but it's still sound and waiting to be cleaned-up and refinished.