While I unfortunately can’t help you with your unique bag specifications, as not reversing the hood of a 500f4 will make for a very lengthy & selective bag.
However I can give you feedback, (just one person’s view) on the Lowepro LENS TREKKER 600 II AW Backpack:
which I have owned for approximately 7 years.
It has the typical Lowepro hallmark characteristics of being very good protection, but I find is a bit awkward & bulky, as the interior seems to have a lot of excess space. (in fairness that is due to the 500f4 shape & dimensions). Also particularly if I hang on the outside a Lowepro TOPLOADER 75 PRO AW (to accommodate a 5DmkIII with mounted 70-200f2.8 IS mkII). I am a petite 5ft 2in female.
However, as we all know camera bags are such an individual preference.
Pardon me, as I don’t mean to hijack your thread/request, but I’m curious about your experience with your Gura Gear bag.
-which model do you have?
-and why won’t it fit under the airline seat? (as I thought they fit FAA 45” guidelines)
This causes me concern as I was seriously considering the relatively new Gura Gear BATEFLAE 32L Backpack:
and had planned on carrying the following when I am “birding/wildlifing” when out in nature…
This on the right side:
-1DmkIII (or 1DX) with mounted 500f4(hood reversed)+TC2X mkII (this is LOA 20.0”)
This on the left side:
-5DmkIII with 70-200f2.8 IS mkII +TC2xmkIII (this is LOA 13”)
-24-70f2.8 mkII, 600EX-RT, Better Beam & ThinkTank Hydrophobia 300-600 rain cover.
I currently have this above content configuration in the very lightweight Lowepro S&F DUFFLE TRANSPORT DUFFLE Backpack:
with some spare bag dividers in centre & around perimeter and a bit/layer of neoprene on the bottom for padding, which results in a great accommodating capacity bag which is pretty convenient to carry when travelling in/out by car or brief hike, or at a rivers edge when seeking out eagles, blue herons, etc;
On site, when the bag is more or less empty it folds/scrunches/rolls and disappears into a niffty "nothing" tube on my back! http://www.lowepro.com/sf-series-event-photographer (see 3:30 minute Lowepro product video)
But feel I would like a bit more padding/protection for my treasured content.
Yes I know such a configuration is a heavy pack (approx 30lbs) and for my occasional air-travel I would have to pull apart/remove a bit of weight from it as a “carry-on” bag to my “personal” bag to insure I can cooperatively comply with most USA domestic airline carry-on bag weight/size requirements, but I have that planned already & done so in the past.
So hoping for your feedback on your Gura Gear bag, thank you.