After a bit of experimentation, I have been able to do this in a satisfactory way. Takes about an hour if you have the parts.
Purchase a 1 dollar red laser pointer. Remove the laser head by opening it up and discard everything else. Put together 3 AA batteries with a switch connected to the head and build a decent stand for it so the beam is horizontal (needs soldering to do it right).
The projection screen is to be placed in a dark area, about 10 to 15 meters away from the laser.
Cut a 1 square inch piece of aluminum foil for the slits/pinhole.
Use pinholes instead of slits. Not only are the patterns prettier, but pinholes are easier to produce.
Create the pinholes by pressing a pin on the foil placed on a paper pad. The spacing of the pinholes should be about 1 mm. The pinholes should be as small as possible. For slits, use a razor to cut the foil - slits should be 1 mm apart, 5 mm or more in length, but as narrow as possible.
Now you need some kind of adjustable hand with a stand and an alligator clip - like the kind sold in Radio Shack to grab wires and things. The clip will grab the foil, and the hand will be adjusted so that the laser beam shines right through the pinholes. It is important that you be able to adjust the spatial position of the foil very finely and effortlessly.
The foil will be placed about 15 cm in front of the laser head. First shoot the beam to the middle of the screen. Then while watching the foil, adjust the hand with the foil so that the beam falls ****with equal intensity**** on the two pinholes (or slits).
You will see beautiful results on the screen. With pinholes, the diameter of the image is about 10 cm. With slits, up to 30 cm long. Try different patterns and dot sizes. The closer the pinholes, the more prominent the interference pattern is. The smaller the pinhole, the IP will be dimmer but sharper and nicer. This exercise is well worth it, and makes a great demo for kids.
Take a look at the photo at this link. This was made with 3 pinholes arranged as an equilateral triangle. Image diameter is 10 cm. The real thing is a lot more colorful.
http://www.geocities.com/zekise/TriplePinhole.JPG
have fun
