Summary

This serves two purposes. 1) Stabilise the two vertical 8mm rods, 2) Direct the filament to the extruder

Instructions

The flat section sits snugly on top of the two vertical 8mm rods.
rotating filament extruder is held on by a top piece, and the whole kept together with a zip tie.

I've uploaded an overall stl, but you'll want to print the 3 individual pieces "-main", "-holder", & "-cover".

The var "PRINT_LAYOUT=true; // or false; " switches between rendering the whole model or the parts. You can select an individual part by prefixing the appropriate section with "!" in the "if (PRINT_LAYOUT)" block.

Adjustments:
You might want to check the separation of your rods at the top - "sepRod=65.5955;" works for me. You might need to modify it slightly.
"rRod=8.2/2" -- adjust this if necessary for snugness.

You could, I suppose, hot-glue it to the printrbot but I wouldn't advise it: It may subtract a few mm from your vertical travel, so you might want to remove it occasionally.

printrbot simple rod stabiliser and filament holder