Tom,

Make a mask with rubylith and tape it to the frame. You can make any window size you need. Use a piece of two-ply mat board or as thick as needed under the printing paper in the printing frame. I have used that technique when printing pt/pd and wanting sharp edges on the print as opposed to showing the coating. I have a very good printing frame but still always use the shim for insurance.

http://www.jeffreyglasser.com/