Conversion of 2009 20" iMac into monitor

Hi all!

New here. I currently have a 2009 20" iMac running OSX 10.6.8. I'd like to get a new 2015 MacBookPro and still use this computer as a monitor for my day-to-day work. I have looked into Target Display Mode, but even though my computer has a mini display port, it seems I must have a 27" iMac to be able to do TDM: Target Display Mode: Frequently Asked Questions (FAQ) - Apple Support

Is there another option I'm not aware of or some kind of app that might make this possible? Thanks in advance for any suggestions!