As I saw the x-ends of the laser cut variant of the Printrbot I thought this could also be done with the plastic variant.
A plate that connect the x-ends tight should bring more stability to the x-end unit. By the way you can give your Printrbot a personal note giving him a name :-)
It is also possible to take my x-end-motor from here: http://www.thingiverse.com/thing:37009
But it is better to use the modified x-end-motor because it has a additional connection plate so the motor can stay on his place. If you take a larger plate that goes between the motor and the x-end the motor will stick out even more. Otherwise the connection plate will lower the x-print area in particular if you have the endstop on the left side. You have to check first if there is room enough between the x-end motor and the x-carriage.
If you want to displace the x-ends to your Printrbot please take account of some limitations I specified here as well: http://www.thingiverse.com/thing:37009