This may not be ideal if you don't also have a laptop but one solution I use relies on having another machine (old or a laptop) to attach the additional monitor. It's a software program called MaxVista.
You install the server side of the program on your main machine and then up to three client instances on other machines. You can then use up to three aditional machines to view your desktop. For example I have a laptop to my left, dual monitors on my desktop and a second old PC with a large screen to my right.
Using MaxVista I have a continuous desktop that appears to my desktops as four monitors in the control panel.