Yeah, this is on my to-do list. However, I have noticed that if I open Big Box from a shortcut on monitor #1, it will open on monitor #1. If I open Big Box from a shortcut on monitor #2, it will open on monitor #2, at least for me on Windows 10.
You can test this by opening up Windows Explorer (the file explorer) to your LaunchBox folder. Drag the window to one monitor and double-click on BigBox.exe. Then drag it to the other monitor and do the same thing.
I realize this is still an issue though and I will get it fixed.