This is the one I bought, maybe I should have included in the package??
L311 ? Compact Long Lens Case
Its pretty well built but it is a little bigger than the 300mm f2.8. They sell two foam "space taker-uppers" to add some extra padding down around the front element and one that fits around the lens right where it meets the camera body to stop things from bouncing around too much. The shoulder strap is actually kind of awkward for carrying and I am thinking the backpack option might be a little more convenient.
My lowepro backpack is also big enough to fit the lens and camera down the center but it takes up a lot of space.
Glad to hear you are having fun with it!