Ah, very good: you have cottoned onto something that mainland Gielinor has failed to understand. The way to make armour as strong as a kalphite's exoskeleton is not to hammer it into shapes that are suitable for humans. The carapace is too resistant, and you will get an armour that is ill-fitting for the human form. There will be plenty of gaps for someone to slip a Keris into.
What you need to do is break the shell into its component fragments. You need to make a kind of kalphite mosaic, then adhere it with something acrid to make it into an armour. That's how you become as hardy as a kalphite.