(Tinkercad here: tinkercad.com/things/4Xq6l4uM02N (beta) and here: tinkercad.com/things/jEY9vuTsFWR (V2) )
Apparently, you can get a 3-foot length of 5/16" (or 8mm) metal rod from Home Depot for $5 (or amazon: amazon.com/Polished-Finish-Precision-Annealed-Diameter/dp/B0050RMXJO ). A hacksaw can cut it in two (eventually!), and the printed parts shown above will let you extend the X axis of your Printrbot Simple to an arbitrary length (other factors considered). It takes a lot of desk space when it runs.
I haven't been able to test the entire length yet, however, due to software limits on the axes ranges. I hope to address this by flashing the firmware mentioned here [update: the thread below has the firmware that eventually allowed me to use the entire range]: printrbottalk.com/forum/viewtopic.php?f=22&t=2980&p=27110#p27110 as per the flashing instructions given here: reprap.org/wiki/Printrboard#Loading_Firmware_.28Windows.29
Now I just need some good print-bed material. Cardboard worked to a degree, but was ultimately too flimsy. I fear wood may be too heavy, as the metal rods themselves already cause quite a lot of vibration...
UPDATE: I found some cheap aluminum round at speedymetals.com/ and a nice, light piece of wood from A. C. Moore.
There are now two sets of files -- one for the Simple Beta and one for the Simple V2