After several years of using two buckets, I recently bought a plain double bucket of the kind available at any hardware or homegoods store. This cost about $9.00.
Since my car is pretty low to the ground, I often work on all verticle surfaces while seated on a rolling stool I purhcased when I got this car. For the bucket, I built a small box out of plywood and 1x3 the same size as the bottom of the bucket, which fits snugly into it. I painted it with several coats of some old paint to keep the water from the wood.
I then bought 4 casters which I screwed to the bottom of the box, 2 of which rotate and two of which are fixed. This allows me to set the box so it does not roll downhill when I am using it on my dirveway, which is a slight hill. (If all 4 wheels swivel, it would roll away on its own when on an incline.)
After reading the above posts, I will make some kind of a mesh grit gard to put in the bottom of one if not both sides of the bucket. I like that idea, although grit from the rinse water has never been a problem for me using two separate buckets.
Now with this bucket on wheels, I can do all sides of the car sitting down, and just pulling the bucket, with soap and rinse water in it, along with me. And the bucket lifts out of the cart for easy rinsing, emptying and cleaning.
Total cost, maybe $15 dollars or so, and it does exactly what I need it to do. I probably spent less time making my little bucket cart than I would have spent driving around to different stores looking for something appropriate.
And I feel so good about myself.
in the washing procedure and you're probably on your way to sterile washing conditions 
