A question for the paint pro's

[daniZZ]

Hi there

I recently pained my first shell and I'm pleased with the result.
But I made a litle mistake. I forgot to leave a small window for the transponder.
We use here the ILaps system and the transponder have to be visible (see the LINK), so I need a transparent window as big as a mobile phone SIM card....and I forgot to leave it clear.
How can I clear a part of the shell to have this window?
I have to say that in the area where I need the window I have only one color (white as the backing for the shell)