Save files are located in \Users\YOUR USERNAME HERE\AppData\Local\CallistoProtocol\Saved\SaveGames

I recommend making a shortcut to your desktop for easy access

Rename the files to PhxManualSave1 (You can replace 1 with any number between 1 and 30) and drop it into the above folder

You will need to restart the game everytime you add a new save so just put all the saves in that you want. Manual saves *should* never get removed automatically

Some saves might be on Maximum Security (sorry) so you may need to change back to Minimum Security