I've done set ups just using styrofoam coated in an epoxy/sand mix (I've also used polyester resin, but getting the mix right is trickier, and the fumes are pretty bad).
I've usually used the two-part pour on epoxy that you can find in a lot of craft stores, and sometimes even lowes/home depot. just mix up a batch, throw in a bunch of sand, apply it to the styrofoam, and dust some sand on top (just makes it less glossy, and wouldn't really be needed if the piece is submerged).
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