I wanted to have the same capability whilst out offroading and found the following
window vent solution for my Jeep JK Unlimited. I also tend to sleep in my jeep. Some thing like this might suit your needs.
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Despite being easy to remove, I leave them permanently in place even when I am not planning to sleep in the jeep as they help to cool the inside of the jeep down on very warm days, which is very common in California. The only downside is a slightly increased road noise whilst driving at highway speeds. They are very secure and well made. I have had mine for almost two years and they are still in excellent condition. They have not aged at all.
The German-based manufacturer, Nakatanenga, also sells some of their products on EBAY and has made versions of these vents for the
Jeep Wrangler JK,
Land Rover Defender,
Toyota Landcruiser J150,
Toyota Landcruiser J8,
Jeep Cherokee XJ,
Nissan Navara D40, and
Land Rover Discovery 1/2/3/4 to name just a few. They also sell an
adjustable, universal version for those models that they do not yet produce a specific solution for. All window vents are sold in pairs.