Don't "Think Outside The Box"
Use boxes instead, a whole roomfull of them. (Returned items unable to be delivered from outer Space)
Some of the boxes have real people in them (who need no costumes) they just move their box around.
Some boxes are filled with deadly creatures from long lost galaxys, or maybe Impalas? (Buick specials?)
Some boxes are empty but come to life with the yank of a long piece of fishline from another box. Sliding across the floor.)
Two fishlines shared by two boxes could make the middle , empty box move left and right, or just reset itself to it's starting position.
Peak holes get hidden in the boxes via black paint numerals or designs.
A few cardboard boxes have inner liners of plywood so once a monster jumps out from his box, he can stand quickly on the reinforced box to gain a height advantage over mere mortals.(To more effectively threaten them.)
The Delux model box could be entirely plywood with real hinges, numerous vision ports, internal creature comforts and ball-bearing wheels to allow for quiet pursuits and speed. This plywood box would be covered with cardboard to make it hide with the rest.
If all the boxes were just painted blackurse the numbering system on the boxes would be numerically similair and confusing to also help hide which box just did "what"?
All of these possibilites run around in my head and sound like a lot of fun to do and to have fun with the patrons, but then immitating what has gone before , Evil Clowns, Hollywood Monsters, ex cetra is pretty well ingrained within many people's concepts of what certain things should "BE"!?