I used a pattern from Choly Knight's book "Sewing Stylish Handbags and Totes: Chic to Unique Bags and Purses That You Can Make". She calls it "Customizable Crossbody Bag".
I should mention that on the versions I've made I changed the front slip pocket under the flap. It's supposed to be two slanted slip pockets and I only made one tall slip pocket. I like how is hidden under the flap. I used the same main pattern front and back piece and cut it a bit lower. Inside there's a zippered pocket. I also added a couple of inches to the gusset pattern since I don't clip the seam allowances when I attach it to the main pieces and I found it to be short.
I like to use the flap to show off a beautiful print fabric and make the rest solid. A niece saw these bags and ordered one with a specific fabric pattern that she likes on the flap. I'll show you when is done. You might get tired of seeing the same bag pattern so I'll switch to another one after that one. Maybe the March Bag from the Bag of the Month Club. I should say that I love it!! I don't only like that is a convertible bag but the shape, the size, the pockets... well, all of it. You haven't seen it? go here and take a good look.
This Chevron Flap Bag is available in my shop here.