RE: Any idea how to do this??
You have two good options presented to you - either use the offset hinges that CARSII suggested, or use the Robart Hinge points (the BIG ones) supported by brass tubing where they hang out of the wood, as madmodelman suggested. Take a look at both and use the one that you think would work the best. I would mount the hinges such that there is no gap when the flaps are all the way up and don't worry about the gap when they are down. If anything, the air flowing through the gap when the flaps are down will be turbulent and give you more drag - which is what you want on a Boomerang. You don't need the flaps for extra lift...
Bob