Here's a link to the copy stand I made from an old enlarger arm and some supplies from a hardware store:
It works great and disassembles to be stored in a closet. For duplication, I use a white acrylic sheet atop a couple light bulbs (focusing is easier with steady light than a flash). Here's my process for medium- and large-format digitization:
I've taken to using a negative carrier as it puts space between the negative and acrylic sheet. This eliminates interference waves that used to appear in some of my negatives when I digitized them.
A flash works, too, but has a narrow light spectrum. So for slides, I use either daylight or full-spectrum bulbs. That provides truer color. For black and white it's much less of an issue and I've used flashes and daylight, both with good results.