Chris,
I use a wooden mount in my car. The one I have is quite fancy and made by someone who modelled it on a commercially available mount. A friend of mine also made a much simpler one for his own use out of two pieces of wood and a hinge, that seems to work fine as well.
You can get some idea of what I'm talking about
here. If you decide to have a go and make one I'd recommend that you avoid using lightweight wood, (you won't be carrying it any distance so it doesn't matter if it weighs a ton). And that you make the top platform wide and deep enough to allow you to swivel your bean bag without it falling over the edge.
Steve.