Check out the narrow masking tapes made by Tamiya (same as Parma) and Testors, using the tube method to get a line is easy then narrow masking tape on the line. You can also get the 1/8 and 1/4 inch tapes from auto paint suppliers iirc. The narrow tapes follow curves much easier without buckling.
I tried the condom, but it wanted to roll up the tip of the NC, and getting it straight was hard. I took a plastic cup, cut out the bottom, and used that. Of course, it works best if you are masking the upper part of the NC.