Metz/Canon help

[cdiver2]

I need help with a shot I'm planing whilst on vacation in five days.

First equipment.

Canon digital XT, flash Metz 76MZ-5 ( I just got the flash one day ago )

I think I will be able to use any lens I want but I am leaning towards a 20 mm ( DOF)

The shot I want to do is taken from just inside a red sand stone cave looking out with only one third of the cave wall in the left hand side of the view finder and the scene out side bright out to the horizon.

So I think I will end up with a outside reading of + F 8 and the cave wall with a reading of - F 2.8.

I have attempted to set up a smaller version of this shot at home, the only way I could get anywhere near the result I wanted was to turn the flash head backwards and let the small fill flash at 1/4 power light up the wall.

So my question is, is there another way to do this shot.

Thanks all