I'd go for a test run of a large bowl full, cos there are other reprocussions with the local cat population if you fill the car up with kitty litter...... they might think it's a mobile toilet
I find that a large shallow tray, like a seed tray or a litter tray works best, you need a large surface area, but needs be no more than an inch deep. Rice and Salt will also do the same, but I think you find that $ for pound you'll get more kitty litter for your buck
Just don't leave the doors open, or Tigger and his pals will be your new best mates cats  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!