I originally tied a short cord around the back lash strap and the small hole in the plug. This keeps the plug attached to the raft so you don't do something stupid like kicking it in the river while you're inflating the boat. Unfortunately, the cord tends to get twisted and it gets in the way... very annoying.
I was able to make this quick and easy attachment point that mimics the keeper on your trusty Nalgene water bottle out of a scrap piece of plastic (this one was from a ziploc container).
Start with some scrap plastic - a milk jug would work nicely as well.
Sketch out your template. Make sure you set the ID to snap into the groove on the plug. I used a 29mm diameter as I recall.
Install your cord of choice.
Voila! Bonus points for fancy knots.