rmann's idea sounds good and so does FranksB's espeically the idea of a mask smaller than the image. whatever route you go some other tips that may help: use a red filter (either under the lens or in the filter stage) turn the enlarger on so you can see the image area and align your mask with greater precision. Also, you may consider using rubylith as a masking material cut with an accurate paper cutter for a straight-edged mask.