Tonight I cut two more holes in the transom. A 1" drain hole, and a square hole for the boomkin.
That makes a grand total of 17 perforations through my transom. Six for the Duckworks motor mount, eight holes for the gudgeons, and three more for the tiller, boomkin and drain! I hope the boat will still float despite all these holes!
Another challenge was how to trim all the planks flush with the transom. I thought about using a saw but was afraid of scratching the surface of the transom and chipping the planks. Then I thought I'd carefully whittle away at them with a sharp chisel. That would have taken forever. I ended up using the Gain Machine
I set the Gain Machine so that the router bit was flush with the bottom of the base, like this:
Then, holding the base firmly against the transom, I used the Gain Machine to route the protruding planks flush with the transom.
The planks were perfectly flush a few minutes later.