Buy some ratchet straps and be bold — you can drag home plywood on the roof of your car. Just batten it down well. (Unless you’re very precious about the finish of your car)
buy a few swimming pool noodles and put them underneath the load, very cheap and effective. Put a twist in the lengths of ratchet straps too and it will prevent them whistling at speed
if you're at a big box store like home depot/lowes, and you find yourself without the swimming noodles you can substitute the outdoor pipe insulation that comes in long foam tubes. or look in the moving supplies area for furniture pads.