Frames to "scan" colour and B&W negatives and diapositives with dSLR
For 35mm, diapositives in 40mm standard frames, 120 in 6x6 and 6x9 format, 4x5".
Buy a square 20x20mm aluminium profile to mount front and back base. Set distance to fill whole dSLR frame for lens you have (macro lens could be required). Screw to avoid shifts when changing negs.
Mount top frame with rubber bands and screw to frame base with M5 screws.
Mount dSLR camera with 1/4"-20 screw (0,5" / 12mm length; search for "Quick Release Camera Fixing Screw").
As a lightbox use some cardbox with diffuser and a flash with cable or radio. This is open construction so flash should give much more light than ambient. Or you can try to cover everything with black cardboard.
Update: BaseFront4ShortScrew.stl added for shorter 1/4" tripod screws (8-10mm)