Obviously it was time to Make Something.
Step 1: Materials and tools
(That was a really short step, wasn't it?)
Step 2: Design
If you can't think of your own idea, there is an excellent source of ideas is this database of plans HERE. They are all intended to be carved from plywood, but you could print one out and trace the design onto your card.
One important point to remember is to round the ends of the wings - it flies better, and it'll do less damage if you clip an innocent bystander.
A note for the sinister;
If you are left-handed, then you will need to make (and throw) your boomerangs in a mirror-image of what I show here.
Step 3: Cutting
Step 4: Taping
Then, wrap the whole thing in duct tape. Try and avoid lumps and flaps that will spoil the airflow around the wing.