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I'd try the Feisol CM 1471 Rapid monopod,
Your 400/2.8L IS + 1DX, on a RRS MH-01 monopod head, with a 2x TC and 600EX-RT flash would weigh in at about 8 kg. The CM 1471 max capacity is 11 kg. I use it with a 500/4L IS and the other stuff, for a total of about 6.5kg, and it's no problem.