The only thing I would be concerned about with shower curtains, is that chickens peck...a LOT. Until my chicks were close to laying age, I had the nesting boxes covered with commercial-grade trash can liners (very thick). They pecked at it a good bit at first, probably just out of curiosity...
Where in GA are you? We are also in GA. We built our pen/coop ourselves as well and I am pleased with the results. Our coop is 8x6 and tall enough to stand in. We have windows with plexiglass that we can slide to.open, or just remove, as well as ventilation at the top. We built the highest...