I bought one of these Allen Co. Changing mats awhile ago at STP for about $5
It folds up into something about 10"x10"x1".
You can fold it into a bowl shape and fill it with a weak bleach/water mix if you are worried about invasive stuff.
I have an old milk crate that I keep that thing plus my boots and waders in. That's what I store it in in the garage and I just throw it in my truck or car when I go fishing. The crate also gives me something to sit on or to put my foot on when tying my boots.