I make palladium prints from 8 x 10 negatives and most of my negatives are cropped at least slightly. Arentz, 2nd edition, p. 92, recommends Rubylith masking paper, but my attempts at mask cutting using Rubylith were not successful. Perhaps I did not have the correct type of Rubylith, but I found it very difficult to measure and cut straight lines in the very curly material which an art store sold me. I kept hearing of golden rod paper as a good masking material. When I asked a knowledgeable salesperson at Xpedx paper company (www.xpedx.com) about golden rod paper for masking 8 x 10 negatives he suggested I use 11 1/8 x 18 5/8 inch AB Dick 360 SC Pre-Ruled masking sheets (Colorlok Precision Pre-Press Products), available in boxes of 100 sheets. They work perfectly for me. (Interestingly, the masking material shown in Arentz's Fig. 10.1 appears identical to the AB Dick sheet, not Rubylith, at least not the Rubylith I had tried to use.)
Briefly, here is how I make cropping masks for my negatives. (In the end an 8 x 10 negative will be taped over a retangular hole in an 11 x 14 masking sheet. I use a Bostick & Sullivan 11 x 14 inch printing frame for printing under UV bulbs.) First I trim the masking sheet to ca. 11 x 14 inches by cutting 4 inches off the end with the catalog number and 3/4 inch off the other end. After trimming, the line labeled "7" becomes the center of the sheet, and I measure out from it in both directions 1/2 the dimensions of the mask opening. Measuring in this fashion guarantees that the mask opening will be centered in the middle of the sheet. Mark each side of the to-be-cut mask opening with a short pencil mark. You do not have to draw the entire outline of the mask opening.
Place the masking sheet on a cutting mat. (I use Light Impressions No. 20198 cutting mat, 24 x 18 inches.) Lay a steel straight edge on a pencil mark and align it parallel to the closest grid line. Cut the masking paper by drawing an Exacto knife along the steel straight edge. I have found that the Exacto No. 16 blade works well and does not wobble like the No. 11 sharply pointed blade. Overcut the corners about 1/4 inch. Repeat for the other three sides of the opening. After cutting the opening, turn the sheet over and cover the overcuts with a piece of 3/8 inch Rubylith tape. One short piece of tape will cover both cuts if placed diagonally at the exact tip of the corner. Overcutting and taping guarantees sharp 90 degree corners. After taping the overcuts, turn the sheet back over and tape the negative to the front (lettering side) of the sheet (non-emulsion side touching the paper) using several short (1-2 inch) pieces of 3/8 inch Rubylith tape---I keep the tape in the clear film edge and don't let it stick onto the image area. It is best to do the taping on a light table so you can position the mask opening exactly (as shown in Arentz, Fig. 10.1.) For printing, place the masked negative in the printing frame emulsion side up and place the coated paper on top ---paper coating against negative emulsion. I leave the negative permanently taped to the masking sheet which makes it convenient for reprinting.
This system works well for me. I get straight sharp edges and perfect corners. For negatives larger than 8 x 10 larger masking sheets would be needed. I haven't had occasion to check with Xpedx to see if larger sheets are available.