This is a very simple method of securing a hip pack to a Jones H Loop handlebar. The attachment method works with many styles of bags simply by attaching a loop near the bottom to hold the bag in place within the H-Loop. At 240 grams this 4 litre rolltop drybag is not ultra light, however it is extremely durable. Newer models are available that weigh 180 grams. The only modification required is to push a knotted loop of paracord through the drain hole of the zippered pocket, a bead or ring to prevent the cord from sliding out is optional. The mounted bag pictured below contains a down jacket and vest, cache battery and cables, and an InReach. The bag also works well as a deckbag when bikerafting.
Waist strap buckle secured inside added paracord loop 
Waist strap fed through from centre of H Loop, over and back to the front of the bag
Valhalla Pure Outfitters carries these Seal-Line Hip Pack bags for less than $65 Cdn. The weight can be reduced by replacing the heavy 25mm (1 inch) waist belt with a lighter grade of webbing and buckle set. This change dropped the weight of the red bag to 180 grams.
Adding an accessory pocket
The red bag has a patch of industrial Hook Velcro attached to the outside of the pocket. It was placed and clamped overnight for the adhesive backing to fully adhere. This patch becomes the platform to attach other items such as an electronics case. The image above has a couple of examples of utilizing different Handy Andy cases for holding devices and securing a cache battery to the Loop Velcro inside the case.