It's possible to either paint the signs, or cut the letters out of a stencil. Some people just print the signs on paper with a laser printer (to avoid the banding you get with large blocks of colour on most inkjet printers).

The best solution is usually to make a prototype with paper and take it or an image file to a professional sign-maker and ask for something suitable for a light box. A thin dark vinyl layer stenciled and stuck to a clear plastic does a good job. Sometimes a thin white vinyl can be stuck over the back as a light diffuser.

We should get some suitable graphics together as a resource for the forum in due course.