[joachim]

The 3kg lens I'm talking about is the Sigma 120-300, which if I took it may get used at up to 600mm. Otherwise, I'd travel light and only use the 100-400.

 

Sand is a minor concern. It should be heavy/wet enough where I'm going not to blow. I might finally give in and fit a rain cover onto the lenses just in case. I have enough bodies that I wont need to change lenses in the field on this occasion.

You need something heavier than a Manfrotto 190 this case and a very solid head.  600 equiv lenses on an SLR are a challenge to get stable.  I would look into a Manfrotto 055 this case, which is all I would be prepared to carry.  But then I am not an experienced tele shooter.  I bought the 190 for my Mamiya C330f and with the low shock medium format TLR it works very well.  It made a huge difference in sharpness to what I was using before.