oven
Inside the car would cut down somewhat on the UV exposure, but it’s certainly not going to be any cooler in there! I’m guessing you might shorten the life of a boat more by storing it inside a car on a hot day than on the roof. Especially a folder or inflatable with glued seams, etc.
I’m guessing a minivan will only swallow an 8 foot boat, and frankly life’s just too short to paddle an 8’ boat on flatwater!
boats on the roof
We are a bit East of you, and I regularly leave the boats on the roof rack of the minivan. They come off to paddle or to rest on the lawn. Figure if it wears out, I have an excuse to buy a new boat.
Important safety tip
The really important thing is to remember to remove the car from the kayak before putting the boat in the water.