One solution for summer cooling might be a separate, opaque panel to cover the roof when it gets sunny and hot. The "Tropical Roof" optioned Land Rover was legendary for its cool comfort in the days before AC. A second roof panel was set an inch above the first, with a gap for air to flow through, so you were always driving in the shade. You could also add some absorbent fabric against the roof, and keep it wet for actual cooling.
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