Originally posted by ruggiex so it may make sense to get an insert
What he said.
A true backpack with a nice insert will give you much more comfort and modularity. If you do a lot of hikes, then you know that selecting the proper backpack for your physionomy is crucial. And you're more likely to find the right backpack if you don't limit yourself to only dedicated camera backpacks.
Inserts are easy to find, inexpensive, and you can remove them if for some reason you don't want to carry your gear on a particular day. I use an Ape Case insert, lifetime warranty and inexpensive, but there are others. Most backpacks have either a top or bottom smaller Pocket, you could measure it and find an insert which fits snugly in that pocket.
For the record, my 15$ Ape Case holds a K-3 (previously K20D) with tripod base plate and a Sigma 17-70 mounted, plus an AF50 flash, DA21, DA40 and DFA100 macro WR, plus some room for a spare battery and some odds and ends.