Use these adjustable surface mount grippers to help level panels on uneven surfaces.

These are also used as a top mount with the non-adjustable variant as a top and bottom mount combination.  By using these parts together, you are able to tension a panel, reducing flex in  the material and providing a more stable mount.

These grippers provide 2 3/4" - 3 1/2" space between the mounting surface and the bottom of your panel.