This pen holder can be customized for any sized Moleskin, any fabric print and any size pen(s).  It does require some basic sewing skills.  Though I did this with a sewing machine, it could be done by hand.

First, measure a one inch wide elastic.  I pulled it just a bit tight, overlapped 1/4 inch and cut with pinking shears.

Then I sewed the two ends, using a zig-zag stitch.

Placed it over the book with the sewn edge on the inside.  Now that I think of it, I could have placed the pocket over this sewn edge to have completely hidden it.

I made a pattern for the fabric pen pocket.  I measured 1 3/4 inches wide and then just a bit below the end of my pen.

Cut out the pattern on my fabric, which was right sides together (double layer)

Sewed it together, turned it right side out, hemmed the upper opening by turning under and zigzag stitched on the machine.  Then I positioned it on the elastic and pinned it where I wanted to sew it to the elastic.

Sewed the inside top edge and through the bottom edge with another zigzag stitch.

And it is done: