I was looking at the sea to summit 20L folding bucket, and found that if you fold the top edge down over the outside of the bucket in order to make it shorter, it becomes much more rigid, and would probably work as a sink. It's about the same diameter as the 5L sink, and would probably work as a hat in the folded position as well. It's lighter than the sink, and holds way more water when fully unfolded. To add to the multi-use nature, I would tap in a piece of tubing in the bottom for use with a gravity feed water system.
If Sea to Summit made them in black instead of orange, you could use the bucket as a solar shower as well, just click a showerhead in where the gravity feet filter hooks up.