Assuming you want to spend less than 50 dollars, head down to your salvation army depot and pick up eight heavy blankets/comforters. Take them to your laundromat, and wash/dry them for safety reasons.
Mount eight picture hangers (big hooks) into the four corners, one hook per side of each corner.
The singer walks in, hangs the last two blankets over the door, then sings.
While cheesy, I must say that it worked much better than I expected and I was able to capture my vocalist's voice in a very dry fashion with minimal room colorization.