Can you string the lamp out to them on an extension cord, put it in one corner and if they want the heat they know where to find it. Those with adult feathers will be fine.

Your box should be plenty, especially if it isn't too tall & it traps the heat down low. We use plastic bins, just personal preference, and we have them hanging around.

I don't like the idea of heat lamps, again personal preference. Too much can go wrong, even with the best set-up.
Brendasue

I too had the same problem. My ducklings are 5 weeks old and my husband wanted them out of the basement 4 weeks ago. I have 2 rouens and 4 runners. Well against my better judgement I put them out last week. He kept saying "They're Ducks, they can take it." So for now at night they are in a 3x4 foot wooden box with a heat lamp with a 40watt bulb. The last 2 nights we had 32 degree temps so I put a 75watt bulb in, shut the lid, put a blanket over the top to block drafts and they were just fine. In fact I probly should have used a 60 watt. When I let them out it was 45 degrees outside and 70 inside the box. Well the little buggers streached and flapped their wings and ran over to their swimming pool and jumped right in. "BRRRRR" They are only half feathered but love that ice cold water. When they do get out of the water,they have the opertunity to go into their nice warm house but opt to cuddle together,out of the wind beside their house. I figure they know best. Their instincts are great, the way they tip their litle heads and look up at the sky at the sound of other birds. I'll never forget the very first day they were outside, I was sitting with them talking and quick as a blink, a little knat flew by and ZAP, he was a snack. "Wow"I said,"Who taught you that?" Tonight it is only getting down to 55F so i think I am going to put in one of those energy efficiant bulbs, that don't give off any heat just for light.
Just a bit of info from my experiance...I have one of those inside/outside thermometers, and when the babies lay on the sensor it reads 85F. I am assumeing that must be their body temp. So 6 of them together can generate alot of heat. I think if the temps don't drop below 50 I shouldn't need any heat at all.
Good Luck They're so much fun !!!