I had this quandary recently (for a DSLR - 400D with Tokina 11-16mm as my walkaround lens) and couldn't find anything I liked that a) fit comfortably without rattling around, b) didn't have straps that interfered with my ruck and c) didn't cost a fortune. I looked at the Lowe Pro Toploader series but it was too bulky for me and the next nearest competition was something like Cotton Carrier stuff (in excess of £100.)
I ended up making my own sling from a neoprene/soft shell type holster and a pair of neck straps to create a chest holster. That held the camera with lens, and I got one of these to hold my other kit (a couple of smallish lenses, couple of filters, spare batteries and memory cards etc.)
http://www.amazon.co.uk/Lowepro-Lens-Case-1-Black/dp/B00009R896The velcro straps slotted nicely over the hip straps of my ruck, though I admit I bought it for versatility as much as anything else - I strap it onto my camera bag to hold extra kit when I'm not in the hills.
I wore the whole setup on a trip up Helvellyn recently in a mixture of rain squalls and winds gusting up to about 50mph (according to MWIS anyway) and it performed brilliantly. Given that it would be showering one second then dry the next, getting stuff in and out worked like a dream. And it cost me about £20 in total. A bit of work involved but I didn't mind that.