Hey Zski
This is the one I use and I have had it for a good long time. Wouldn't be without it.

I have a small mesh bag into which goes the soap/scrubee. I use the cord on the mesh bag and "hook and loop" it around one of the handles of my sink. That way everything stays together.

This is a great size for 1 or 2 people. It is also the right amount of water to rinse soap off (away from the shore) from your person. Fill it up, set in the woods, go for a swim, soap up and grab your handy dandy granite gear sink. The loop at the bottom is a great aid for rinse off as well as drying upside down.

I keep mine on the outside of my duffel secured by a biner and straps against loss. It will stay wet for a time, even upside down so keep that in mind. And yes, a reasonably flat surface is good.......Not to big, not to small, just right :thumbup