Like Foam EZ says you'll be just fine. I drove a 12'3" Clark from Santa Cruz up to West Marin, about 100 miles, on the roof of my '87 Golf 2-door. My ass was puckered the entire ride but all was good with the the blank when I got to the barn.
Hell I put a 16ft shaped 1.5 lb eps paddle board on top of my Nissan Murano. just cinch it down with comealong rachet tie downs....Man up, its not going any place.
...you can put whatever and be the lucky guy and nothing happens...however, shits happens; I use a bunch of foam per year; mostly arrive as a bulk in a big truck but sometimes they arrive in cars, on cars; so I m not talking in the other comment to transport a blank and be the lucky guy...but yes, do whatever you think is right. 1 longboard blank + light density on top of the car, no matter if you put a bit of cardboard is a total risk to buckle the foam; last one for me was a guy that brought me one about a month ago. Arctic foam, yellow density; Toyota truck; 240 kms ride at certain speed I guess; buckled under the straps 5/8" deep.
You are such a bummer!
That which can be assorted without evidence was read in an illegal magazine.
...retarded look over other to your entertainment. Go back to your 1 or 2 dings per year.