Customizable filament guide designed for the Prusa i3 Berlin with aluminum frame. I designed the guide so I could use a loose filament spool (http://www.thingiverse.com/thing:39564) mounted behind the solid metal frame.
The frame width is adjustable, so it should work with other frames. You can specify where the guide opening should be relative to your frame and the arm length and angle will be automatically calculated.
The design is adjustable for the width of your frame. For the nominal 6mm thickness of the Prusa i3 Berlin 6mm made a nice firm grip (printing at 0.3mm layer thickness, 0.6mm shell, 20% infill, 0.4mm nozzle).
Measure the distance from the frame to where you want the filament guide opening to be and plug those into the x and y distances in the customizer. The OpenSCAD code will automatically determine the necessary arm length and angle.
You can make a version for a spool on the left or right side of the frame (this puts the arm away from the spool so the frame grip braces better - I found that with a loose grip the guide wants to fall towards the spool).
You may want to print another guide so that the filament stays off the edge of the frame (http://www.thingiverse.com/thing:117720)