Create a clown circus show or a clown freak show, which makes the clowns the focus of the set. Think about the typical features associated with a freak show and incorporate those into the design. Use enlarged posters of the clowns in the show, but make the images darker. Hang the posters in old-fashioned or antique frames, complete with scroll work and carved edges. Create areas on the set for different freak show acts, such as a snake lady, bearded woman or strong man. Scatter fake body parts and fake blood around the set and let your clowns wander through the space. This setup makes it appear as though the clowns attacked the other freak show acts.
The underground set is a dark and dank set, made to look like the sewers. The story behind the set is that the clowns were banished to the sewers and mutated into odd and unusual creatures. Round pieces of metal serve as the grates and entrances to the tunnel. Paint the walls shades of black and grey and use the same shades on the floor. Fake spiderwebs draped around the walls and fake animals make the set look even darker and more dirty. Use smudged makeup and muted colors on the actors portraying the killer clowns.
One of the scariest ideas is finding something horrific in an unexpected area. Create a killer clown set that works around this idea by building your own fake carnival. Create fake storefronts and booths from painted pieces of plywood with support beams behind each one. Use bright colors, including red, yellow and blue. Craft a few working booths, such as a ring toss and put actors inside those booths. It's unexpected when a killer clown runs out from the shadows. Alternatively, make the set in the same way, but scatter the set with trash and fake spiderwebs. It still maintains the happy feeling, but also shows that something isn't right.