For as small an area as your looking at a small rooftop unit like the Cub mach 8 would likely be enough. But the opening size is still the same as a 15k unit.
Since your van will be parked when you ar using this, have you considered a portable AC? 6000 to 9000 btu models are readily available. Get one if possible that uses two hoses for heat transfer and you will find them more efficient. The hoses can be run through a piece of luan or plastic mounted when you park in a front window opening.
Cost should be in the $200-$300 range for a single hose model, slightly more for a twin hose.