I also have the Nature Trekker and use it less often now when walking the hills where we live in Scotland.
I was looking for a pack that was weatherproof and versatile enough to carry my photographic gear as and when I wanted a particular set of kit, namely macro, wildlife and landscape. I eventually decided on the Lowepro Flipside 400 AW which I can kit out with different types of equipment and it is not too heavy. The big advantage for me is the ability for it to swivel around on the hip belt and I can easily take out my gear, a big advantage when climbing the hill in the rain with all my Paramo gear on.
A link is here:
Lowepro - Flipside 400 AW
I cannot vouch whether it will take the Bigma, as I do not own one. It certainly accommodates the Canon 100-400mm L lens and hood, plus a variey of other lenses and 5D.
Paul.