I make a light canvas pouch in colors to match the bent. They're lined with 3/16" insulation foam and have a bubble wrap bag that the hydration pack fits into. The insulation foam hasn't been installed on the bent in this image.
When the bag is in the pouch and filled, the pouch is smooth. The hose runs down the underside of the seat to the front hook & loop seat pad strap. It's out of the way and easy to access. The hydration pack is accessed through a pair of zippers on the top for easy filling.