Hello!
My first thought was to take a narrow strip of tape (about 1mm) to mask the edge of the round window, and then take something liquid (Maskol, Revell Color Stop or something like that ) to mask the center. But then it occured to me, that a tall "chimney" made of wide masking tape (~20mm) would also do the trick. For extra safety you could stuff some tissue down the open top. Hope it helps, good luck with your builds and have a nice day
Paweł