Hi Ray,
Excellent idea, I have been thinking about how to do this for some time. To make the bottom footing of the bracket more secure I was thinking of using a two part rubber potting compound to mould around the foot. You could use some modeling clay to make a dam in the pen holder, spray some release agent onto the exposed surface and pour in a two part mix of potting compound.
What is the model of the holder you are using?
Steve