The easiest way to put black borders on a print is to make a mask smaller. Now, instead of trying to put it in the center, do this two step process: Put the mask in the upper left corner right at the print edge, and expose white light. Then slide the mask to the bottom right corner (diagonal), and expose again. Real easy to do and get the borders even.
You know, like all the best ideas, this seems to simple and obvious now, but has never occurred to me before. Thanks.