Just a couple of piece of live rock will work....

Rubble will too, but imo holds a lot of detritus... And makes the fuge harder to clean..

I put 2 good sized pieces of LR on a little stand made of eggcrate.... I can see what i need to clean and critters have no problems running up to hide when the lights come on....

I love mangroves and have one thriving on my bare bottom fuge...