The launch and descent after balloon burst can be very chaotic. To prevent the BalloonSat's innards from being bounced to death, either bolt equipment into place or use a filling material.
Bolting can be done with #2-56 hardware to keep it light, but you'll want washers to make the head of the bolts larger. Try using plastic (20 mil thick?) that you can purchase from hobby shops that sell model trains (or other construction type hobbies).