Since you already bought a raft, I won't say that for fly-in stuff you'd be better off with a cat. (Or did I?)
Anyhow, my adventures with frames and fabrication are mostly contained in a few threads on the forum—search for 'Games with Frames', 'Drop Seat Mount', and 'Ultra-light Oarframe'. They include specifics on sources & materials, and how-tos. I'd say the best compromise between weight and cost is to use chainlink toprail tubing (steel, 1.315" OD) and Hollaender SpeedRail 1" IPS joints.
A couple general thoughts: the worst
place to break a frame tube, especially a weight-bearing member like a seat tube, is in the center— that puts max stress on the joint. For an idea on good places to break a frame, take a look at the Sid's Sports frame parts (from Cambridge Welding, I think) on the Jack's Plastic Welding website http://www.jpwinc.com/sfrprt.gif.
These are steel and meant to mate with chainlink toprail tubing. The weight of a frame built this way is quite similar to that of an aluminum frame by NRS— their alu tubing is larger with thicker walls and their LowePro joints are quite heavy. Another dodge (covered in Games with Frames) is to use 1" EMT electrical conduit for your corners, pinned with machine screws.
Tinkering takes time. If you're in a hurry, I'd get a frame from an experienced builder.
The trips sound great!