I needed to build a small one not only to get better visualisation of my negs, but also to conduct film speed and development tests. I got the original idea from Build Your Own Home Darkroom by Duren and McDonald.
It was dead simple: I got a 7' x 1' piece of 5/8 melamine, cut it down to 8'' width. Next, I cut four sections to make a box (mine is 12'' to a side). Next I cut a bottom out of MDF and then placed a ceramic light socket with an 8'' circular fluorescent fixture. I cut two pieces of perspex for the top and sandwiched a piece of tissue paper in between to diffuse the light. Parting stops (mouldings) are used to hold the perspex in place.
It was really simple to make and I did it with some scrap wood that I had laying about the house. It took around 1 hour to make.