I've looked around for external battery solutions for my H4D-50 but not liked what I've seen. Having an extra pouch with a battery and a stiff firewire cable to mess around with is no fun out in the field.
So I bought a $40 ebay battery, a Duotec band (a form of hook-and-loop) and a firewire-to-dc L-plug from Leaf. The DC-to-DC cable came with the ebay battery, here I've been lazy and not shortened it just folded it up. The battery holds power in the back for almost five hours (passive), so it's okay.
I was expecting the Leaf L-plug to point down rather than up, but as Hassy has the port on the opposite side as Leaf (I think) it points up, but it's okay, the cable mess gets out of the way.
The Duotec band is very strong, so I had to use very narrow strips to not have it sit too tight. The battery weighs only 180 grams (6.5 ounces) so it does not need much strength to sit. The mounting does not look too good but it works and is more practical than any other solution I've seen. As seen in the attached pictures you can mount the back in both horizontal and vertical orientation, and the sliding back fits down into my backpack as before. In other words, the external battery won't slow you down, it's attached all the time.
Haven't used it in the field yet, but should work fine. You don't need to worry about the Duotec glue either, you will be able to remove the strips from the back without leaving marks.
On the minus side is that the weather-proofing is not great , but I can live with that, and you could make some hacks to improve that aspect too.