You could expand your canvas as Walker suggested, but doing so is time consuming. And the background doesn’t always fit the available film page size accurately. This can result in unwanted black lines at the outer edges of the film. My 3880 jammed more than once trying to print that close to the trailing edge of the sheet.
I use another technique that’s fast and works flawlessly.
The secret is to first invert your positive image changing it to a negative in Photoshop. Save your file with a new name such as “image-neg.tif”
Place the negative file in the Print Tool window as you normally would.
Then, click the checkbox labeled “Negative” in the lower right corner of the Print Tool interface. This activates the associated Print Tool preferences.
Click on Prefs… and choose Background Type “Black”, and Border “None”. Make sure the “Flip for Emulsion Side” is checked as well. Click “Done.”
Now Run Print using the appropriate PiezoDN curve.
When you print a neg using this method, the negative original you placed is automatically converted to a positive image by Print Tool (and flopped emulsion side up, though it doesn’t show in the interface) and converted again back to a negative by the PiezoDN profile.
Remember that old principle from math class? A negative of a negative always yields a positive. When you string an odd number (in this case, three) negative conversions together you end up with a negative which is what you wanted in the first place.
But in the process Print Tool adds a 100% black background that adjusts perfectly to the film size no matter where you position your image on the page. And if you ever want to reprint at a different size, you don’t have to create a new file. Try it! It’s much faster than fussing with it in Photoshop.