All you need is an ohp, large prism and two sheets of card or books that can be used to make a slit on the top of the ohp. You’ll also need a clamp, stand and boss to hold the prism in place, a G-clamp can be used with the right ohp.
There is a Physics Education article that goes with this demonstration, November 2004, Overhead projector doubles as a classroom ‘rainbow machine‘Inge H A Pettersen.
The prism takes a little bit of messing about with as it needs to be at the same height as the mirror at the top and catch the beam. Gives a ‘rainbow’ that’s usually well over 1m in length. The books just make a slit by placing them on top of the ohp glass. Having a thin slit gives much better definition of colours; usually approximately 1mm works.
An interesting exercise is to get pupils to stand in the spectrum with one eye shut and move the open eye through the colours. Not only are the colours seen very beautiful but they can also see that the light is being split up at the prism rather than the spectrum just being a pattern produced on the screen.