I looked at the Etsy pattern...which I would call a 'tunic'. The one made up in the bright yellow fabric gives you the best chance to see what it would look like without a belt, and I agree with Marianna that a slim belt would not be a good idea -- or any belt, for that matter. A more subdued solid or a nice print would look well, in my opinion. Maybe not as wild as the print in thebig photo but whatever you like that fits your lifestyle and taste.
Have you read Angie's wonderful articles about how to dress the body types? They are fantastic and I highly recommend them. I believe the shape you described is an 'apple', so that's what you should look for.
ETA to find these posts about dressing the Apple shape, use the Search tool (top, right-hand of this site) and choose 'The Blog' and 'For All Time'. Enjoy!
