This idea was not mine, but a fellow club member who had "engineered" the idea of tree supports for his 30 footer. I was impressed and thought it would certainly work for the cape dory.
1. Find a couple of sturdy nearby trees for the task and tie off some leftover halyards which don't stretch under pressure.
2. In my case, since the dory is smaller than my friend's yacht, run the line to your winch, wrap well, and crank a bit to increase tension, then tie-off on a secure cleat. This is your main support so do it right, taut but not overly tight. The boat is balancing and you're pulling on both sides of the boat to keep it balanced.
3. And just in case, I added some mental assurance in always working with one bunk up while sanding and applying pressure to the hull from the opposite side. During painting I will push both down and emplace two wooden supports (where ladder is in the photo) on either side under the rub-rail to keep her from favoring a lean in either direction. I'm probably more nervous than I am smart, but these halyards probably can handle the dory just fine!
Doesn't look like much drama from this perspective does it?