Sam's Club sells (or used to) a refrigerated bag to keep your ice cream in on the way home.  I use that.  It is easy to store without fish.  Keep a couple of blue ices in it and it should keep the fish relatively fresdh until you get home.  Of course, you wouldn't want to borrow the one your wife uses.  Once used for this purpose, she won't ever want it back. 

I do what Cosign does.  If you forget to bring the bag in, you'll wonder why it's suddenly so hard to paddle...<LOL>.  Only problem I have with that is the fish only stay alive a few hours or less that way.  I've toyed with the idea of putting a circular float in the bag so the fish can swim around a little, but storing it while paddling would be a pain.  And, I'm to lazy to actually build one to see how it'd work..