you dont need a box, theres a few in my club that drop things from our funflys, i like dropping bananas coz they explode on impact!
we have a servo mounted outside the plane with a star shaped servo horn, an elastic band is glued/screwed to somewhere on the plane and stretched to hook onto the servo horn (channel 5 on retracts) flip the retract switch and the servo turns and lets go of the elsatic band. it may take a bit of fiddling but it is the lightest easiest and cheapest way to do this.
almost anything can be placed under the elastic band.

a good 'bomb' is two styrofoam cups filled with flour and turned lip to lip and taped together, these look good when they are dropped.
if you want a picture of the mechanism email me