Lens Case for 300mm L IS f/4
I use a Lowepro lens case 4 for mine. It's kind of bulky, but well padded. Comfortably fits a 300/4IS and just barely fits a 300 with an EF1.4x attached. The biggest advantage for me, though, is that I use some of the Lowepro belt pack systems (Off Trail and Outback) and the various Lowepro lens cases all attach directly.