See, look, it matches my Sunday Afternoon Capelet!
I just came up with the idea and the pattern myself as I thought of what kind of purse I would want to carry around all day at a convention while in costume. There is an open slot in the back of the back (for quickly sliding handout flyers and business cards), plenty of space along the top of the front flap and on the strap for pins and your name badge, and ample open room inside for a wallet, phone and camera. I added an extra slip-in pocket inside as well.
I saw this pink and white button at JoAnn's and thought it was adorable and it just happened to match my pink-and-green plaid fabric. I'd been looking for a good place to put it, since the cloak didn't have a place for it!
I added the ribbons on the side to make it fun and flirty and nod to the ribbon on the front of the cloak.
Here is my concept sketch and pattern piece:
I used this one pattern piece as the pattern for all of the fabric pieces just by folding it in different places or turning it over. If you want to make your own, just pick a width and height and make it squared and symmetrical. The folds determine how wide and how tall the purse will be.