Hello. My preferred boat for rowing and sailing right now is an 18 foot, y-stern, cedar canvas, prospector canoe. I am just finishing two wannigans for this canoe with the same intention you have. One sits behind me in the boat and the other up in front. ( I have removed the middle thwart to make space for rowing and sailing.) They are to be secured into the boat and provide flotation and storage.I intend to secure them by lasshing to my gunnels which are open and to thwarts. I put air bags in the ends of the boat and Optimist sized air bags either side amidships. ( I did get five foot roller launching airbags but they proved to be too big) I sit on the floor to  sail and sit on a small stool to row. I wanted to experiment with some new constuction materials so I got some Divinycell  foam in sheet form and laid it inside the hull of the canoe to exactly conform to the inside hull shape. ( Spread some plastic sheeting there first) Then I epoxied it and it held the shape. Fiberglassed the foam inside and out, added marine ply to the ends and top and viola- wannigan! They seem sturdy enough to sit on. I always go ashore to sleep but have thought about sleeping in this boat. I have enough space to lie down on the floor of this boat if I raise up one of the wannigans which will then help to support a fly of some sort. Good luck and have fun!

I love the idea but think it would have to be rather large to accommodate sleeping. Dry bags and a canvas Dodger may be more practical.   Given that I would probably recommend moving the location forward to lie between thwarts 2 & 3 giving you better balance under load and room aft for you to station while under sail. . . Unless you only plan to row in which case the aft location may be better. 

JP