20° (68F) is a sufficient temperature for duxling when it is dry and sunny outside. As said, i had more issues with panting ducklings than with shivering ones. Just observe them, when they are running around, exploring everything its perfect. When they are huddling , even piling up together to sleep that's fine. They will tell you loudly when they feel cold and they will start to pant when it is too warm.Yes we do have a rubber mat on the bottom so lifting everything out should be fine.
Very glad to see such little ducklings doing well in a natural environment. Hopefully the weather here really will turn warmer again and we'll be able to do the same. We had a period of 25-30 C in mid April and right now we're just hoping to avoid below-freezing overnight (with mixed results).