After buying and building lots of other cages, I've just bought some big parrot cages (heavy, powder-coated metal), and my igs seem to really like them.
I bought sale items from Fancy Paws at
http://store.yahoo.com/moravia/dibica.html, and they sent the cages on semi truck to my house for free.
When they're little, soft or wire mesh cages are good. When they get big, heavy-guage wire or bars seem to work best. Mine didn't like solid wood sides or plexiglas doors - they want to climb on every surface.
Hope this helps!