I take a dip every day I backpack. Simply do not use soap and then there is no need for any device. I also find that solar showers (where I backpack it is not really warm) may feel nice at first, but as soon as the cold air and wind hits your wet body you end up just as cold as simply jumping into the lake. The key to tolerating a cold dip, is to do it immediately when you arrive in camp, when you have excess body heat. It actually feels good then. DO NOT jump in a cold lake if you are already cold, particularly just before bed. You will have a really hard time re-warming. In this case, take a dip mid-day, while you are hiking, so that you warm up while hiking.

The only washing I do that uses a little bit of soap, is once-a-day face and hand wash at night. I do this in my 1L cooking pot, away from water sources. I use the dry facial wipes (Dove) that are actually intended for makeup removal. The "suds" are gone after one use. The wipe is sturdy, almost fabric feeling, and that is used to wipe out pots after cooking and when it gets too yucky to reuse again, it goes in my plastic garbage bag (I carry out all TP, wipes, and any other packaging from food).