If it was a case for one lens with the body attached it may not be too bad for a backpack.
You can calculate the proper size for your camera here...
For 1 body and a lens the size of my 200mm I'm not thinking the case would be all that large.
It IS however a SOLID case.
Not sure how comfy it would be against your back hence attaching it to the outside of the pack.
My case is a 1550 more like a small suitcase to allow for my 500mm and a few other lenses.
Definately too big for your purpose but still has room for an extra body with another lens attached.
Pictured is my K100 body with 300mm attached in the 500mm slot and a 50mm with hood attached set in vertically in it's spot.
My cams frequent wet/fast open boats and saltwater alot, as well strapped down on the front rack of a four wheeler for day trips through all sorts of "stuff"
Never a worry for me.
It's a GUESS but a case to fit one body and lens...I would say about the same if not less than the wieght of your cam
Accessibility = 2 latches, flip them open open lid and viola,cam at hand as pictured above.
When the lid is closed camera is completely/snugly encased in at least 1" of foam all around.
The foam in the case is precut into squares that you remove to customize to whatever you want.
Relatively easy process no tools required....measure TWICE-cut ONCE
If you happen to know anyone in law enforcement or paramedics/search and rescue/fire dept.
I'm sure they could show you a case similar before making a purchase.
If you didn't I'm sure if approached they would willingly oblige a look anyway.
Hope this helps?
Pentax 18-55mm,Pentax 50-200mm,Sigma 17-70mm,Sigma 70-300mm, Sigma 50-500mm