This one is dead simple. I have a car. I have a Rubbermaid tub that I fill with
water and a little laundry
soap. The tub is filled to about 2/3, then the dirty clothes are added. This much water will slop out of a container, even with the lid attached, if filled with only water. The clothing works as a slosh baffle, and the liquid stays in the container. I like to do this on hot days. The cold water quickly heats up in a car that has the windows rolled up. An extra temperature
boost can be realized if I empty my
solar shower bag into the container. Nothing is very far away in Victoria, so I'm not putting on hundreds of miles per load. I just go about my business for a day or two and then dump out the container. Sometimes I refill it with cold water, for a rinse cycle. Sometimes I have a house that I'm demolishing and the bathtub can be filled with cold water for a rinse.
My car doesn't have air conditioning. The washing machine and shower bag absorb some heat, so that when parked, the car doesn't get quite so hot. After the washed clothing hangs outside for a short time, I drape it over the passenger seats and hang some of it by the belt loops on those hooks that are meant for hanging a suit. This turns the whole vehicle into a giant swamp cooler, when the windows are down a little. This low tech system works pretty well.
Damn it, I can't find a picture of the Rubbermaid tub. You know what they look like. Here's the shower bag sitting in the back window of the vehicle. It gets so hot that I often have to add cold water before using it.