I cut up some pink insulation foam, using my Proxxon Hot Wire Foam Cutter, painted them a light tan, then I applied The Dip, Royal Walnut tint.
These are completely modular, and they are designed/sculpted, to show a natural cavern routed by water, rather than hammer and chisel. They are fast to sculpt, on my Proxxon: slope the wire around 5-10 degrees, off vertical, then wiggle the foam as you cut it, to achieve the scallops.
If you want a more chiseled look, try something other than the wiggle technique, by making them more squarish, with the cutting. Then, texture them with a crumpled ball of aluminum foil.
Here is my first attempt at making modular caverns, using pink foam, cut on a band saw -- did not have the Proxxon back then. I took brown paper grocer bags, cut into strips. I then crumpled the bag strips while wearing pig-skin leather gloves, to protect my hands. I then flattened out the crumpled paper, which gave it the textured, chiseled look.
I then employed Low Temperature Hot Glue (High Temperature Glue will melt the foam…), to apply the strips to the foam pieces.
I found a texture/modeling paste formula on YouTube, to save money. I applied this paste to the tops/bottoms of the foam pieces, to mate the paper edges with the foam edges. I used a stiff, stippling brush to form a texture in the wet paste, then I let it dry.
I finished the pieces using The Dip -- Royal Walnut, as already detailed. These have an obvious chiseled appearance, and texture. They are also an incredible amount of labor to create! They are incredibly modular, easy to use, and they look fantastic sitting atop my randomly sponge painted, Harbor Freight Floor Foam Pads. Here is another close-up look at the finished pieces[, in game use.
Here is one of my favorite photo's: Link; "No flesh… Gonna' have to do this 'Old School', I guess." In D&D, Green Dragons breathe Chlorine Gas (not fire), for those wondering about the Dragon's comments… The foam mats were cut to 1/4" deep, on a Table Saw, to create the one-inch grid. They were then painted using craft paints, and a natural sponge, as an applicator. I used around four different colors, applied while wet, to allow some blending. These cavern pieces clearly show the stippling brush effect on the homemade texture paste, used on top and bottom of the pieces. Some of the first pieces received an S-pattern of Hot Glue, but I found that I was using up Hot Glue way too fast, spending way too much money on it! The homemade texture paste cost around $3 USD, total, to mix up a huge batch. And I prefer the ease of the texture paste, as well: slather it on, stipple it, let it dry, then paint, and Dip.
I hope you find this informative, if not useful. LOL! Cheers!