I don’t carry my 64 in the car, but I have hooks like these on my passenger seat, that I hook my 40oz off of. The flex handle might work but I have a paracord handle like this on mine.. The setup works pretty good, I can easily reach behind the back of my passenger seat.

Sure, here are the links to both:

Queecord (includes paracord handle and carabiner clip)

Gearproz HydroCord (includes paracord handle, lid, 2 straws, cleaning brush, shoulder strap)