I used two puppy playpens for eleven Runner ducklings. Each had 8 panels 2ft wide. I used 6 mil plastic to make a liner that also turned up at the edges for about ten inches or so underneath, with old towels for bedding. I suspended ceramic heater bulbs at one end, and had an old sheet draped around to prevent drafts.
Note - I started with a 4 ft long Rubbermaid tub, but at two weeks they outgrew that, then I got one puppy pen, two weeks later they outgrew that, then we finished up with the 2 pens end to end. That worked, though it got a bit tight toward the end...but by then they were going outside for quite a while during the day (supervised).