You've got to have a pen to go with that Moleskine. Here's my solution.
Step 1: Cut the elastic to size.
I used 1 inch wide elastic although you could use wider for a larger pocket. It needs to have some overlap like I've demonstrated in the picture. The final circle needs to be small enough so that it fits on the front cover snugly.
For this project, I cut the elastic 21 inches long, so that it would fit a large Moleskine (8 inch x 5 inch).