I'm not condoning it, but a guy I ride with used a 45mph street sign to make a seat pan for his Harley. I think they're stainless steel, plenty thick, easy to cut and come off with a 1/2 socket... so he tells me.
A bunch are aluminum here. My sister's husband Rich used a stop sign to make his but got lucky and got an older sign. I have a bunch of old signage laying around but they are all aluminum too which sucks:angry7: