To ensure the two hinged glass plates on your Greystone by Furrion 2 Burner Gas Cooktop replacement glass and metal base lay flat, it is important to install all components correctly. The glass plates are designed to rest with support from the burner grate, not solely on the frame. By inserting the burner grate over the burners after placing the glass and metal base, the glass plates will be properly supported and lie flat, resolving the concave appearance. There are no additional missing parts required for leveling the glass. If you experience further issues, verify that the hinges are not damaged and are correctly aligned, but typically no hinge adjustment is necessary once the grate is in place. Unfortunately, specific installation instructions may not be included with this replacement part, so following this placement method will ensure proper fit and function.