We got this collapsible trash bag holder online for about $6. We love it. It fits right on the dash (between the vents so it doesn't blow around) and you can put grocery sacks in it. Super cool.
It lays flat unless you need it open. We have it for small things (tissues, wrappers, etc.) The big stuff won't fit, but at least we won't have stuff blowing around all over the place since there's no AC in Humboldt.
It seems to be well-adhered, as well. We'll see how it stands up to heat in the van. So far, so good!