The cheapest way is simply to place a negative (-) spherical lens in front of the output aperture of the projector. You could try a -2.00, -4.00 or -6.00 diopter lens depending on how much divergence you require.
Probably the best place to easily lay your hands on these lenses is your local Optometrist/Optician/Eye Doctor. If you have a 1 hour Optical Lab in your area just ask them to sell you an uncut lens. These are usually around 70mm diameter, they may also have lenses that already have a multi anti-reflection coating applied. I doubt you'll easily pick up glass lenses nowdays, but the plastic ones are just as good.
Cheers
Jem
P.S. Why don't you fill in your profile, or at least give an indication as to where in the world you are?, who knows, I may be able to help you with your quest
Quote: "There is a theory which states that if ever, for any reason, anyone discovers what exactly the Universe is for and why it is here it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another that states that this has already happened.”... Douglas Adams 1952 - 2001