Mendel Pen Holder

I had problems using the original pen holder on my Prusa, see http://apapageek.blogspot.com for details.

So I used the original STL file as a template for reworking the design in OpenSCAD. Along the way I made a few changes to make using the pen holder easier.

a) Redesigned the top bracket so that there are channels to guide the rubber bands and a loop to hold them captive. Now if there is a balance problem, or when you take the pen out the bracket does not try to orbit the room.

b) M