I didn't try the box over the head thing in the end (Mark convinced me it was a bad idea, and since I do get mildly claustrophobic perhaps he's right...) - but I did try singing under duvets. My daughter piled on 3 duvets, 5 pillows and a 4 ft teddy bear before we reached what she considered was a reasonable reduction in sound.
Then we tried just shutting some doors & covering the doors with a duvet. This she reckoned would be OK so long as no one had a headache. Thanks kid!
On balance I've decided that its easier and cheaper to decide just not to worry about it too much. The sound of distant wailing can't be TOO bad can it?

Kris