Or you could use the world around as your supplie source you would be surprised what you can find out there especially for sci fi because imagination has no bounds.Heres an example
http://img1.photobucket.com/albums/1003/cplchilly/?action=view¤t=4f6ed558.jpg
The main body is slightly modified vacuum cleaner tube the wings are acrylic sheet and the rest is the parts bin. Now I know this isnt a tank but Im using it as an example as to what can be accomplished with Items around the house or workplace etc. You should go to the sci fi forum and get some Ideas from woody, boybuddho and some of the others cause these guys are great at this and can offer you a lot of Ideas also.