A flat glass bottle. Much on YouTube on how to do this but most of it is a joke. I can believe that a kiln is needed, not sure about the salt. One instruction says to use a toaster oven for the better part of a day. Uh, I don't think so. Sounds like a good way to burn out your toaster oven and heat up your house.
If this item wasn't so heavy, I'd find a way of putting a message on the bottle and sending it to Spain.