What's going IN the prop, and how large are you making it?
If it's a bucky skeleton, they weigh 23-24 pounds, so you'll want some solid materials.
If it's a blucky or other lightweight body, you can get by using lighter.
Something you may want to try, and admittedly, it's a bit unusual, is fiberglass resin and cotton cloth over some decent gauge wire.
Old flannel sheets may work even better. The material will pil up and make a lot of bumps.
(remember to use rubber gloves of some kind when doing this)
Here's how: cut strips about 3"-4" wide, as long as you need - take a length of wire, bend it to the shape you need (circles, or whatever) set the wire on the strip of cloth, and fold the cloth over the wire, leaving it in the center. Take your fiberglass resin, and liberally coat the cloth - work it in with your gloved fingers, and let dry.
These will actually harden up to a pretty good strength on their own - and will have a rough texture that's ready to paint up to an old, rusted, weathered finish.
I've made the ribs of my "flying ghost" using this method, and they're actually quite strong (and only a half inch or so in width).
