Someone already mentioned conduit...I've used that for both forks and sissy bars.
Hubcaps...and more conduit.
Someone mentioned coolers...my kid and I used one for this thing.
I used sheet metal, masonite and an old curtain to build this thing.
Beer bans were mentioned...I used one to build a Moon tank for my gasser bike.
I used a piece of wood, some marine vinyl and a shoestring to make the 'chute for it.
I used my old garage lights to make a headlight and a tail light for my Halloween bike last year.
I mounted a battery powered strobe light under the seat.
I used black cats from a Halloween cake to make valvecaps, and I attached chains to the back that dragged on the ground.
I used a folding beach chair for my beach cruiser.