I solar warm water in my 2.5 liter platypus, suds up with dry-soap facial wipes, and dump the warm water over me. This seems to work. The facial wipes are small enough that I do not "over-suds" so 2.5 liters is enough to rinse off. I just pour water out the opening - no need for a shower attachement. Probably about 90% of the time I simply jump into the lake or river. I only do the warm water bath every 5 days or so on long trips or occasionally if it is too cold out. Even with warm water, you are going to get cold standing there naked and wet! I actually think jumping in the lake even when it is ice cold, is actually faster and less uncomfortable.

I also find that if I plan a long hiking day, a bath at lunch break is sometimes better than waiting until late in the day when the sun goes down. Even if you get chilled, you will warm up with more hiking. And it really refreshes you for the afternoon hike.