Welcome to AA!
I used slate to make "levels" in my tank before. My LFS
has the most amazing 125 gallon FW
tank with a very long wall of stacked slate, most of it about 12-15 cm. With slate you wouldn't have to worry about having substrate up top, but I'm not sure you want a solid wall.
1 yoyo loach, 2 polkadot loaches, 6 cherry barbs, 2 turquoise rainbows, 1 chinese algae eater, 2 harlequin rasboras; java fern, java moss, hornwort, Asian ambulia