I have used Cub Scout Pine Wood Derby race car weights. I found them at my local hobby shop, but check them out here: http://www.pinewoodpro.com/weight-sets.htm

I used 2 strips of twelve (.25 oz weights (6 oz total weight) and stuck then inside on the hull floor; and against the walls; left & right sides; so they were under the DMD Units and the battery (but not under the speaker box).

John