something like this would work out well for u
http://www.ebay.com/itm/VINTAGE-LASE...item3393929d8f
would prolly just cover that 20x40 area
these are also neat
http://www.ebay.com/itm/Hot-sell-Min...item5ae89d700b

^^ i buy those cheap lasers and pull them apart to build low power animation lasers out of them for small spaces

this would also work good if u dont want to buy something really old
http://www.ebay.com/itm/American-DJ-...item4161315930