The Fisher Space Pen fits snugly across the top or bottom of the pocket Moleskine.  The pen is pressurized and writes well on the Moleskine pages – even upside down.  I was going to buy a piece of elastic and sew it to keep the pen and notebook connected, but since I keep my Moleskine in my back pocket, I just put the pen in first with the notebook on top – and so I don't need the elastic.