A Savage 24 12G over (there are a lot of choices).
Would proberbly be good. It is reasonably light, can be broken into 2 parts for backpacking has a polymer stock with a hollow butt.
I think they may be out of production but you should still be able to pick them up.
I think I would get a .308 bottom barrel myself for ammo availibility all around the world.

I also think I would have a 20G and 410 gauge sub caliber adapter machined for the 12G, a one inside the other job, ie use both for .410 and one for 20G to make sure you can shoot whatever you can scrounge up. I might also be tempted to machine a 22LR adapter for the 308 barrel. Yes accuracy will be rubbish beyound 10 yards but it could get you out of a hole if this is a real survival (Amazon jungle) type situation.

Regards