Are you sure that you wish to leave a Mac 128K (or even a Plus) running unattended such that you need to use a screensaver?

I have seen compact Macs with serious screen burn -- Macs used in educational computer labs which had been left on for weeks -- but none of my survivors suffer from it. If your Mac already has screen burn, there is little that you can do about it. If your Mac does not have screen burn, you will have to abuse it for a long time to create a problem.