yep, steel, concrete, all types of wood, plastic, fiberglass resin, granite, acrylic, and yes even tiles have been done before.
The point in any of that is for a rigid, smooth surface, but using some materials can add an unwanted resonance at certain frequencies so for SPL that can work well, but for SQ not so much.
I use the good ol fiberglass resin/sand/portland cement mixture for a solid, resonant free material.
Oh and also hexagons and octagons have also been widely used with just OK results.
Believe it or not, simple rectangles and squares have performed the best in cars, with some key angle shaping on the interior corners.
Would you be so kind as to share with us how to blend and apply this mixture?