Can I use a cooler to keep my laptop cool?

Yes, but with important caveats. A cooler slows heat transfer, so your laptop might stay cooler for a couple of hours longer than in direct sun, but if left in a hot car all day, the internal temperature will eventually equalize. The bigger risk is condensation — putting a warm laptop into a cooler and then into air conditioning, or adding ice packs, can cause moisture to form inside the electronics. For short trips only, keep the cooler in the shade with the lid cracked; a reflective sunshade or insulated laptop bag with a mylar blanket is better.