Roswell craft had spider silk type of synthetic covering similar to Kevlar now (The Day After Roswell) Corso

The last article reminded me of what Corso said about the covering of the Roswell Craft and how it was more plastic and web like like a spiders web woven with incredible strength and able to stretch and not break. So, likely this
 Aluminum polymer composite 
likely is moving in this direction too. So, imagine using a large enough 3d printer solar powered by the sun in space and weaving your own crafts that can withstand unknowable at present stress loads to travel to the nearest star.

So, all you would have to do is to get the 3d printer parts and something like Aluminum polymer composite up into space then assemble it (because whatever you built would never have to withstand earth's gravity if it was only going to travel between planets or stars and never land on a planet. So, it could be constructed in very interesting ways. However, using our present propulsion systems if you were not using the magnetic fields of planets and suns for propulsion through some kind of gravitic or magnetic propulsion likely you still would need metal parts for the propulsion system to keep it from melting the aluminum polymer composite in space from the combustion of present propulsion systems that were not gravity based or magnetic in nature.