I had the idea of making a common accessory rail for the Printrbot PLUS, and each of these blocks should press down onto the 12mm smooth rods. The blocks are designed to be strapped together with an 8mm threaded rod, one nylock nut in the hex hole, and another nut on the other side. This also allows you to adjust the spacing between the smooth rods (I found it varies quite a bit from the top to the bottom, causing the X axis rods to shift).
Anyways, this is still a work in progress, and is an expansion from my previous stiffener blocks.http://www.thingiverse.com/thing:149670.
There's a photo there showing the setup.
On the back of these blocks is a huge hole to install the spool holder. There is one on each side, so you can strap on 2 spools. The huge round thing is the spool holder.
The little thingy is a filament guide, and should clip on to the 8mm threaded rod pretty firm. There is an M3 screw hole there anyway if you really want to clamp it down. The blocks also have an M3 screw hole should you wish to really bolt it down to the smooth rods.
The tabs under the main part of the block and the hole is if you want to install some wiring to accessories on the 8mm bar. The tabs are for zip ties. Later on I was thinking of adding a webcam mount attachment and some lights or something cool like that.
This is still a work in progress and I am yet to actually make it myself, But should get a chance to pretty soon. If you give it a go get back to me with feedback.