I used a quilted under shirt that I found at a thrift store. I have not been able to find such an item new, though. Most of them are polyester, too, which is a pain. Eventually, I was able to score a cotton one, though. I removed the sleeves and then dyed it the correct color. You could spray paint a polyester one, I suppose, but you'd never be able to wash it, and I've found that spray-painted fabric tends to remain kind of tacky and gets yucky after a while.