The easiest way to build a copy setup for film (slides, transparencies and negatives) is to lay a lightbox on the copy stand and then put a negative carrier on top of that. This video from Digitizing Your Photos shows you how set one of these up (including how to make sure that the camera and the film are parallel to each other.)
I cover several other setups for copying film in the book, but this one requires no special tools and can be made wth stuff that is commonly available at camera shops.