Their are a number of great ideas for making a sleeping platform in the dory. The best of these is by Silver Salt and is essentially two bench seats that can slide to the center to create the bed.

My own implementation came before I knew about Silver Salt's bench seats and is a collapsible PVC frame with a close-cell foam mattress on top. All of this gets stored beneath the thwarts and can serve as flotation when not set up for use.

Here is a link to a Google Photos album that contains pictures of the platform   

   And here is a link to a thread in the forum about my setup.

https://www.clcboats.com/forum/clcforum/thread/36411.html

   Been noodling a similar concept but with battens  laced together, one set spanning the rear and another for next 2 thwarts ,would be easy to stow when  not in use 

   Silver Salt's solution (sounds like a quack medicine, doesn't it?) has sliding bench seats spanning the middle thwarts. Those are just the way you would want them for sailing. When camping, you slide them together to form a narrow bed and you fill up the space behind the rear thwart with the camping gear you have inevitable brought. That gear provides enought support for your lower legs and feet. He's got some pictures of this setup posted somewhere in the forum.

It's what I'd do, if I had to do it over again.

I wrote a long blog post with ideas for Northeaster Dory camp-cruising modifications, complete with drawings:

Modifying a Northeaster Dory for Camp-Cruising

In the linked article John says:
 Sitting up and out to one side on the bench seat, the skipper's weight exerts powerful righting moment. Great! you say. Except in light air, or a sudden lull, when the lightweight boat lurches to windward. Your own weight is sufficient to capsize a boat in this weight class.
That is is unequivocally, exactly and hilariously true. Add some powerboat wakes and Bob’s your uncle - if you want to swim. Although in my experience it was more of a gentle slurping of half the Rhode river into the cockpit.

Thanks for the plug Mr. Birch. One unintended and surprisingly useful bit of the sliding bench seats is being able to slide one of ‘em more or less to the center and sit astride it. I’m able to move freely between the second and third thwart tweaking the trim as necessary for point of sail.
During the head scratching phase I was very skeptical of a system without leg support. In practice I’ve never been uncomfortable. If you’re overnighting there is plenty of gear to support your legs.