This free standalone application - no installation necessary - unmounts the BoxCryptor drive after about 10 minutes of inactivity (the real time to unmount the drive can be higher).
Just copy the file BoxCryptor-Unmount.exe anywhere and start it, nothing more to do. If you need to close it, right-click on its tray icon. To auto-start it, create a link in your Windows autostart folder.
The executable is digitally signed by me.