Bag for 7D & Sigma 50-500?

I am after advice for a safari trip. I am after a bag that will take a 7D with the sigma 50-500 mounted. Reasonable padding and dust proof are important criteria. Ideally a shoulder bag that can carry this 'standing up' - not too keen on rucksacks.
I like the Billingham 445 but it does look big. I will only carry the above, a 15-85, spare batteries and few bits and pieces.
The lens is 22cm long! Many thanks.

I use a LowPro Dryzone 200 Back Pack and it takes my 7D with the Bigma 50-500 installed
on it in the center of the pack and leaves room for carrying my 70-200 F/2.8 L IS II, 17-50 F2.8,
30 F/1.4 and the 10-20 plus all my cards some filters and a couple other things. And it's even
more than Dust Proff. You can seal the bag so that it's Water Proof.

--Evan