Kind of out of my usual range here...but assuming that you'll be using VC paper, then either a combination of two filters (N.D. and green) cut to size to fit the contact frame - or perhaps find a dark green glass filter...(the kind used for "development by inspection") and use this in place of the glass in your contact frame - assuming this filter would have enough density to deal with the U.V.? (the green in either case to deal with negatives that are likely a bit "punchy" in contrast for silver gelatin printing).

Not sure about the above, so it would be great to have others with more UV-box experience to chime in here!