Put 'em in cooler after pre-heating the cooler with hot water (for about 10 minutes). Drain, put the ribs in (still well wrapped in foil so no holes). I usually put an old blanket (my "smoker" blanket") in on top (less air to keep warm), close it up.
I've done that and held them for 3 hr. They were too hot to handle bare-handed when I took em out. You'll lose "crispy" bark but that's the price for keeping them warm.
Some might suggest putting them on the grill for 2-3 min to "crisp" them up b4 serving. Remember, they're already cooked so don't overdo them if you crisp them up.