We use a 1 gallon weed sprayer with a kitchen sink spray nozzle added. One gallon is plenty for my wife or daughters to shower and wash their hair. I can easily shower with half a gallon. If it's a cool or cloudy day, we heat some water in the tea kettle.
However, I recently looked at a boat that had a couple of empty, half-gallon plastic liquor bottles in a plastic milk crate up on deck
. They were the sun showers. The owners had used them for years.
Simple is best. I've taken those plastic bag, purpose-made sun showers on sailing trips and found we just didn't use them.