I do carry in the mini trekker with hood reversed and a 2x tc attached but it is a very tight fit -ideally you need something a little bit longer than the mini trekker.I was intending to carry it without the hood attached, but reversed. Probably with a 1.4, or 2x converter attached.
I have been looking at some bags HERE they do some very reasonably priced stuff inc a couple of holdall type shoulder bags that will take a 300/2.8 with Camera, extender and hood all attached and still have room to spare - not necessarily for walking with in the field but good for storage and transportation.