As with human packs, not all makes of dog pack will fit all dogs. You may have to try several different ones to find one that fits the lovely Tussah. Make sure there's nothing like buckles in sensitive areas that will cause chafing. If the pack doesn't have removable fleece sleeves for the underbelly straps like the one in my avatar, you can make some.

The pack I used on the late Hysson (in my avatar) is the Ruffwear Palisades, quite a bit pricier. I bought it because I wanted a pack with detachable harness. It's far easier to remove the pack at rest stops without having to undo the harness, and you can use the harness separately to assist the dog in tough places, or even as a safety harness in the car.
Excellent article on dog packs. Note that this article is 5 years old, so some of the links and info may be out of date.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey