Can I Get My 300 mm In A Backpack?

Hey Everyone!

I've seen some good threads on bags here, but none that address my particular question.

I've started to use my Canon 300 mm f/2.8L IS a lot. Typically, I'm on my bike while I use it, trying to get from one part of a race course to the other ahead of runners. This means I am carrying, at a minimum, that big lens, a big camera body (1K Mk III), a monopod, and the usual clutter of cards, cleaning cloth, etc. Usually I have at least one other lens, too, maybe a 24 – 70.

I would really like to find a super-secure backpack to hold all this. I want something that goes over both shoulders and stays centered on my back. I tried the kind of bag that goes over one shoulder and then cinches around my waist, but that just doesn't feel secure to me--and it won't hold the 300 mm.

Any ideas? I don't mind a big bag, or even an expensive bag, but I want something tough and secure.