All you will need is a 6-8" piece of 550 cord and a cord lock. You can match the 550 cord and cord lock color to your ruck if you so desire.
You could just use 550 cord, however I like the idea of adjusting the cord lock to keep a tighter hold on the hose. The 550 cord also allows for better freedom of movement than those plastic aftermarket retainers I've seen.
This will also work on other pack/rucks that have attachment points on the straps. For those that want to add a multi-purpose system can purchase one of the LED Cord Locks seen below.