A waiver will not hold up in court. Like those waivers at motorsport events, yeah, they are junk too. There is no way to get away from gross neglect. If you can't build a good product, don't sell it. You can however include instructions of some sort that say that the part is to be used for a specific purpose, for rocksliders say a step to get in, and if anyone does something dumb and kills someone you can just say they were not using it as directed.