O.K., let's get started.Take a piece of cardboard and draw a triangle so that it fits in the box and so that both triangles still have about 3 inches in between them. Measure about a 1/2 inch away from the triangle. Then cut the cardboard with an exacto knife and trace it to make a second piece. Cut very shallow on the line where the triangle is to bend the cardboard pieces so that they can "stand up". (Look on the picture because it might be confusing.) Then put the triangles in the box and adjust it if it doesn't fit.
Now, once you're done with that, glue aluminum foil to the triangles (not the parts that bend!) and let it sit overnight. I added aluminum foil so that it acts as a reflector for the LEDs. Make sure that you leave plenty of room around the triangles so that when you're done, bend the aluminum foil to make a border around the triangle.
Poke 9 holes in each triangle so that they aren't too close to the ends.
I did it a bit differently, but this way's probably easier.