Of course, the ultimate would be to get a second-hand jump seat from a late 
model Vanagon. Between this idea and the bean bag idea, you may consider 
using a cooler with a padded top (available at most marine places). The cool 
storage is, well, really cool. Putting it behind the front passenger seat 
worked for me, and you could even rig a seat-back and use the passenger 
safety belt bolts for attachment and additional safety belt. I'm sure it's 
all highly illegal in a mandatory seat belt state, but it'll get you to the 
store and back in some peace and quiet.
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