Fake Rock Structure Alternatives

[Deabrua]

As the title suggests, I'm looking for alternative ways to go about doing the fake rock structures. From what I've seen, most people use styrofoam to create the bulk of the structure, expending foam to fill in, then level out with grout before painting and sealing. While I wouldn't use this, I came across a clay substrate and was wondering if there's smth like so that would be okay to use? Obviously any necessary precautions would be taken as I would with sealing the structures made from foam.

 

[chrsschb]

Nothing wrong with clay or other hardened materials, you just wanna make sure you seal it.

Keep in mind some kinds of soft woods are bad (like pine).

 

[Deabrua]

Main reason Im not going with what I initially found is its intentionally made for digging, any specific product suggestions for something that could be sculpted & sealed?

The tank itself is mostly birch with a single oak board for supporting a shelf. All of it has 3 coats of water based poly-somethingicantremember (the food grade sealant that everyone suggests).