Grab a cloth surfboard sock for your board, it'll minimize the sand/mud in your car, prevent wax from rubbing on your car interior and catch any melting wax, prevent yellowing of PU boards caused by UV rays, act as a towel in a pinch,  offer a bit of cushioning protection, and looks allright too.

Surfsock dimensions are roughly 180 x 50 cm and should suit most kite surfboards or regular surf shortboards. The pictured board is 5'10" and fits snug lengthwise.

These come in one of two colours: white and black (pictured) and white and blue.