FWIW, when I was just carrying my 200-500mm and an attached body I used a decent day hiking/ski touring day pack with a rear panel entry to protect and carry the camera. I do the same thing these days to carry my 500 PF and camera. The specific pack I use is an Osprey Kode 32 which is probably a bit bigger than you strictly need but when I'm in the backcountry I usually have some rain gear, food, water and a few other items so I like the room in this pack (plus I use it in the winter for ski touring).
I believe they make smaller versions of this pack and I know there are many out there like it. It doesn't have perfectly fitted compartments like a photo pack but it does have a very secure and dry inner compartment big enough for your lens and camera combo and you could always drop something like a Shimoda core unit inside if you want the camera specific partitions.
For pure photo packs I've mostly seen versions that are larger and would carry a full camera kit or very small versions where your 200-500mm with body attached wouldn't fit. The new Shimoda X-30 looks awesome for a rugged rain proof photo/adventure pack but again it's probably larger than what you need based on your description.
