I can set "infill" when 3D printing an item. That means, I can set what the inside of an object looks like. standard is 10%. That means that inside, there is a honeycomb like shape. At a setting of 10% infill, the inside is made up of 10% PLA (the plastic material I use to make objects) and 90% air. PLA is a rather solid material as you know from your radiators. I can set any value up to 100% infill, which would mean there is 100% PLA inside and 0% air.
Option A) Try to cut it with a fine blade.
Option B) Use a Dremel ON LOW RPM. But don't cut along the actual line you need to cut. Instead, cut a smaller area and sand the rest.