You mean a negative carrier? All enlargers have them for varying sizes of negatives. Designed for specific makes/models of enlarger and usually made from metal or plastic, not cardboard. Some enlargers can handle 35mm only, some 35mm to 6x6, and some LF enlargers can handle 35mm to 4x5". The enlarger lens focal length will need to be suited to the size of your negative. Usually a 50mm lens to print 35mm negs, an 80mm lens for 6x4.5 or 6x6 negs, 105mm for 6x7 negs and 135mm lens for 4x5".

It may also be possible to make a cardboard insert with a 35mm opening to fit into a negative carrier for 4x5", but I would be concerned about film flatness and dust on the neg from a material like cardboard. You want negative carriers to be as rigid and clean as possible.