Why improvise a solar heated shower? Nice units are readily available. My experience is that they work very well. We did a field project in which we filled our showers as we left camp for work in the morning, returning to pleasantly warm bags in the late afternoon (summer and fall conditions). This procedure worked well for four of us, eight days consecutively for about three months. I got to the point where 2 1/2 gallons per shower was more than enough. I have also used the showers by heating water on a stove and mixing to the correct temperature. They are great items that are compact, workable, and fine morale boosters.