Carry bag for 500mm f/4

I bought the 500mm for a while but haven't been taking it out to use much because none of my camera bag can hold it.

The original Canon trunk is not really good for transportation and I have been forced to use a Samsonite suitcase.

Are there any suggestion of good carrying bag for this lens?

I normally take this lens to hiking (birding and wildlife) so it needs to be a backpack.
I do not take a lot of gear so i prefer a smaller/lighter bag.
(my planned gear for wildlife shooting: 5D mark2 + 500mm/4 + 24-105L + tripod)

The Lowepro flipside 300 seems to be okay.
http://products.lowepro.com/product/Flipside-300,2083.htm