Script to unmount mounted image automatically after some time
Here is a way to accomplish most of this using a simple Automator script:



Save the script as an Application. Place on desktop or dock. When you want to open the encrypted disk image, simply drop it onto the script icon and it will mount it, pause, and ask for confirmation from you to close the disk image after a certain amount of time has elapsed. The only thing here is that if you choose not to have it unmount, you will need to manually unmount later. You can also remove the confirmation prompt altogether. Then you will simply get an error message if it tries to unmount while still open (which should also stop the script from running).