I think once you put it in a chassis it won't be much fun carrying it through the bush, maybe something carbon fibre??
Completely agree. I missed the carry through the bush requirement in the orginal post. Any chassis stock is going to be cumbersome and absolutely unpleasant to carry. Before you buy anything, I'd want to figure out what the weight of the entire system is going to be. A near 20 lbs. weight is going to be a limitation.
Why not consider a McMillan A1 or A1-A3? You can get them in a number of fill weights and their size would compliment your desire to keep the whole thing as compact as possible.