I decided to try still another method to make a wall around the inlet. I took some 3/4 inch Styrofoam, and created some variations in the wall using a heat gun. Then I coated it lightly with some Hydrocal, primed it with gray paint, and added a touch of black paint. Finally I added ground foam and lichen using the 50% white glue and water mix. The other side wall was done using the normal Styrofoam, plaster cloth, and Hydrocal The walls were then pushed into place with no tape or glue holding them. For the inlet, I used various shades of blue paint to give it the illusion of depth. Here are the pictures.
3/4 inch Styrofoam with surface deformed with heat gun,
some hydrocal added, primed, and painted
Green paint, ground foam and lichen added
Around the corner, the wall was done with foam and hydrocal.
It turned out to be a good place to place the NAOGRR Billboard.
Inlet was painted, and sand and rock added.