check out gura gear - their bataflae 32L is huge and will easily swallow all you equipment. Also rugged and very lightweight.As you said: It's huge. Half of it will hold a 600 f/4 or 800 f/5.6 without camera or a 500 f/4 with camera.
OP mentioned 300 2.8. So a Bataflae 26L would be a better choice. It will hold the 300 2.8 with camera attached in one half and the other equipment in the other half with room to spare...
P.S I do have 26L and believe me it's BIG. You do not want even bigger than necessary...
I tend toward larger bags, so (1) I can keep equipment easily accessible and (2) take that extra items (water, sun screen, sandwich, ...) with me.
Well this thread is about OP's needs
300mm f/2.8 IS II (with lens hood reversed - body attached is nice but not needed)
70-200 f/2.8 IS II
24-70 f/2.8 II
16-35 f/2.8 II
1.4x & 2x III TCs
Tripod on back/side
Misc filters, RRS pano slide, small first aid kit, snake bite kit, lens cleaning kit
The 26L holds all of them with room to spare.
In addition he didn't require water, sandwiches and sunscreen. Not that I would put them in a photo bag that carries so expensive equipment. Murphy's Law is always active ...