Here's a photo that I just took of fish extenders that mfabre and I made for our upcoming cruise.
I used the directions linked above. I made my pattern 9" wide by 28" long for the front/back and the felt used as an interfacing. Hindsight . . . I would use 9 x 26" for the felt and not place it at the top so that you could make a casing to run a wooden dowel at the top. The felt makes it too thick, especially if you use flannel for your material as I did.
The pockets were 6" x 9".
I used a 3/8" seam allowance and 1/4" for basting.
The designs on the pockets were iron-ons that I made from clipart that I found online in the Creative Designs forum here at the disboards. The name idea is from Pat in that forum. The "what happens at sea stays at sea is a triplefigs design.
What would I do differently . . . not use black material but matching the red was impossible.
It was really easy!
None of these ideas are original and are a culmination of things I've seen on the disboards in various forums.