This program will automatically restrict the mouse cursor to your main monitor, as long as a full-screen program is open on it.
The program consists of a single 8KB .exe file.
You can download it from my website:
https://files.thecybershadow.net/AutoMouseLock/
Just run it.
The program has no interface. As it is designed to only activate automatically, it should not get in your way, and you can leave it running. To close it, just kill the process from the Windows Task Manager.