If it gets too hot, my rabbits sit in carriers in the house and go back outside in the evening when it cools down. My rabbits have runs where they can lay underneath their hutches on ceramic tiles where it is cooler. The frozen water bottle idea works great, but they do sweat, so I put them inside litter boxes. I don't use regular tarps for shade because they hold in the heat. Menards sells cargo tarps that are like screens that allow for airflow, and the heat can escape. Mosquito netting might also work for shade, but I haven't tried that one yet.