I'd plug it with an engine.
My C26 had a bracket off the back, but the engine well worked much better. The well affords easy, immediate access to the engine controls and prevents cavitating - the bane of stern mounted outboards. But, if you'd prefer to have the noise and exhaust of the outboard on the transom I would still recommend keeping the well open. The prime reason is because it is a great cockpit drain. Those little (usually clogged) drains in the corners will not empty the cockpit very quickly in rough seas. At anchor, it provides neat access to the water for washing, watching critters, and disposing of food scraps by feeding the fish. An open well will splash occasionally into the cockpit while underway. All I did to prevent this was to cover it with one of them square floaty cushions. I considered that well to be one of the best features of the boat. I liked it so much that I'm thinking about adding one to my 18' weekend cruiser.
