MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
This impressive design was one of the first I have found when searching for a spool my Printrbot Simple Metal, many other designs also utilize the guide arms and are situated at the top. However, I believe that these 'top' designs for Printrbot are NOT a good idea because the center of gravity is raised; this seems to be confirmed by @proooo in a comments below.
So after more searching I have found an alternative that places the spool on the back - my modification to it can be found here: http://www.thingiverse.com/thing:2034472