Summary

For those whichever - always or here and there ;-) - have to carry their Printrbot I created a carrier.
Even though the Printrbot is pretty small and not so heavy it is a bit cumbersome to transport because the only stiff parts are the bases connected with the rods.

Instructions

There are two versions:
One to mount the carrier outside of the bases if the rods are long enough "Carrier outside"
And one to mount the carrier between the bases if your rods are to short "Carrier inside".
To fasten this carrier you will need a screw 4 mm x 55.
You can also use a 4 mm x 55 threaded rod.
If you have to mount the carrier after the bases already assembled you can use the "Rod Clamp" both-sided to lock the carrier in position. Or just take an endstop holder and cut off the bar as I did (the yellow ones).
Additional one can use the threaded rod of the grip to mount a vertical rod to fasten a fan on it, as you can see on the left side.

For printing I used 5 perimeters and 20% infill as you can see on the second picture.

Edit 26.2.2013:
I added the files:
Carrier outside.FCStd
Carrier inside.FCStd
grip.FCStd
These are created with FreeCAD, a free software which you can download here: sourceforge.net/projects/free-cad/files/

Carrier to carry the Printrbot