First, cut out a few thin strips of foam, about 1/2" to 3/4" high. Feel free to use off-cuts and scraps, as it doesn’t matter if the foundation you create with these strips is even.

Assemble these puzzle pieces together to form an ellipse and glue them down to a circle of hardboard. Let this assembly dry before you move on to the next step!

Once your foam has fully adhered to the hardboard, you can start shaping it with fine-grade sandpaper. Sand down the outer edges of the pool until they seem to merge with the wooden board. The join doesn’t have to be seamless, though. You’ll be concealing the joins with spackling to make one continuous surface. Also, round down the inside edges of the pool.

Finally, spackle the edges and add some details. Try making teeth-like shapes out of foam. Use spackle to create an organic “lip” to your pool. This example has two Rippers from the Tyranid Warrior sprue. They appear to be relaxing after a long, tough day of slaughtering Mankind!