Leave the topper on if you have large vent windows. It wouldn't be good if you ran into a cold rain downpour. While you are hauling the bees they should be clustered. If they are running all over the cages and loud they are to hot. You will have to drive with the back window/door open and the front and/or side vent windows open. Do not stop during the day when it is sunny for anything more than putting on fuel. Carry a garden hose with and if the bees appear to be to hot you can find a faucet at a gas station and wet them down to aid in cooling. Make sure the package cages are bundle with lath to prevent them from blocking air flow from each other. if you decide to go without the topper don't tarp the bees if you run into heavy rains. That would be worse for them than getting wet.