Many surfactants/detergents (naturally derived or not) are not readily biodegradable. They will wash into water systems and can (in sufficient quantities) upset the local microbe/algae balance. Thankfully most phosphates have been removed from detergents for a few decades now.
In areas that have high human traffic, I try to contain and dispose of the grey water in a responsible way. In remote areas with little human impact, I spread the water out on some deserving flora. If storing the water is not an option, I use a biodegradable soap. These will only break down in the soil, so I dig a cat hole, and pour the water in, then bury. This keeps it in the soil to break down, instead of washing into the local watershed. In delicate alpine areas this can be the best approach.
In my van I have been using a plastic hot water heater drain pan.
https://amzn.to/2NjLPat They are available in various sizes, and are very durable. I just strap it to the side of a cabinet, and pack stuff around it.
The collapsible unit is in an interesting idea. I think that a mortar mixing tray would be roughly the right size. While not collapsible, it can do double duty as a storage container. Or get two of them, and just nest it inside an already filled one for ease of usage?
