And depending upon the interior material of the rental vehicle again, look to velcro to secure the PVC carrier, or alternatively, you can never go wrong using 2 cinder blocks to secure the carrier. And then you've got two seats or rod holders with them there cinder blocks as well.
What else? Dental floss, signal mirror, solid first aid kit with blood clotting agent (I once had a fairly significant hook removal event and the clotting agent was beneficial - http://www.amazon.com/Adventure-Medical-Kits-QuickClot-Silver/dp/B001BCMLWQ/ ), antibacterial soap for all those wonderful fish y'all will be removing from your hooks.
Depending on bodies of water and surrounding land you may consider a snake hook. http://tongs.com/p_snakehooks.aspx Multi-purpose they are and consider the 45" hook.
Depending on the states you're traveling through a firearm may not be on your to bring list, but you should consider some level of protection and deterrent. If you're unable to carry a firearm consider bear spray.
http://www.bearspray.com/ Quite powerful. I've had it halt a charging black bear and more than one "neighborhood dog". Invest in the bear spray, not the personal pepper spray. The bear spray has a higher volume of spray, is stronger than personal pepper spray, and shoots a further distance than personal pepper spray.
