Well since I use a roll of 5 inch paper I have to cut them in my darkroom to whatever size I'm looking to mess around with. I have a mini rotary trimmer there as well as a large rotatrim to use afterwards. I found the fastest way to make them was to use a smaller piece of glass and not use a contact frame. My method gives me edge markings and a black border though.

But I remembered a solution to your problem and looked it up just to double check. In Lootens on photographic enlarging, chap 11 photographic boarder printing, he discusses a simple method of creating a custom sized easel out of cardboard and two strips of card stock in the corner to act as stops for a piece of glass laid on top. Then he makes a mask with that piece of glass ontop using rubber cement and black paper over the whole piece of glass. Your desired photo size say 5x7, would be traced onto that black paper leaving room for the boarder you want. The centeral part is then cut out and the cement cleaned off leaving a simple glass mask. He goes on further about how to make another mask for black boarders with the same rubber cement and black paper on glass method by leaving the center section and cutting a thin strip around the edge.