Aluminum is very easy to work with. You can get a 1/8" sheet of it at your local hardware store that should be long enough for a Pred machete. Or you can use 1/8" balsa wood and when you have it carved to shape, give it a clear coating to seal the wood before painting it.
For your handle, fabricate a bone out of paper mache or super sculpey and wrap it a bit with some leather or brown fabric strips.
How about card, it's super easy to work with and if you get the compressed sort< like you get with A4 writing pads > It takes PVA into it and becomes rather like plastic.
Like my cannnon. If you're wanting a quick and easy prop, then I think this might be the route you want to go, as sculpting something like a blade would take a bit longer if you're not familiar with working with clay, then you'd have to mold the sucker up in a flexible mold and make a resin or fibreglass version.