Swamp cooler. It's a standard accessory at VW shows.
It uses ram air to spin a fiber cylinder to pick up water from the bottom, then the air flows through the damp fibers for evaporative cooling. The better ones have an electric fan booster so you don't become dependent on airflow around the A-pillar.
They don't work well on older VWs without flow-through ventilation or pop-out back windows. The only place for the air to exit is the driver's door window. Here's one with plenty of flow-through.
