Coming up with a compact light weight shower option for backpacking is going to be tough, I guess a perforated cap on a bottle would be minimal, I just can't imagine trying to get wet, soap up and rinsed off on even two liters of water, in my opinion for anything short of a week long trip wet wipes work for me.
If car camping I use a compact shower unit that has a small battery (4 double A batteries) operated pump that has a filtered hose that sinks to the bottom of a three or five gallon bucket, the other end has a five foot long small dia hose with a hand held shower head on the end.
I usually hang the head on the ladder on the back of my van, you can preheat the water or in hot weather shower with air temp water, it works great, we've been using it for about seven years and it's still working great, the batteries usually provide two showers a day for three to four days depending on how long you shower, I don't remember where I purchased it, it was either REI or Eastern Mtn. Sports, $19.95.
