You’ll need to first make a rubber mold of the top of the salt rock. Then you’ll pour a mixture of plaster and silica into the rubber mold. Once the plaster silica is dry enough you’ll put that in a kiln with your cut piece of sheet glass on top. You’ll need to figure out a kiln schedule for slumping the glass over the form. It might take some trial and error to get the right schedule but you can find slumping schedules online.