Occasionally you need to mask off a perfect circle, and wouldn't you know it, there isn't anything of the size needed.

Here is one method I use, and for an example I have picked the chest piece on the Moebius Iron Man to show how I did it.

First I select a hole in my Verlinden 1/72 scribing template (8.3mm dia in this case), and attach some Tamiya tape to an old glass picture frame, and tape the template to the glass.

and using a brand new 10A scalpel blade at 90 degrees to the glass, I run it around the template.

Remove the template and use the blade to lift an edge, and lift it from the glass using tweezers.

Then place on kit and burnish down using a toothpick. Voila.