Atomix

Inspired by the Atomix desk toy from 1972: 

For calculating the number of balls on a given area I used this website. On 100 X 100 mm around 2800 balls 2 mm diameter ocuppies the full area. Therefore I used ~2000 balls to have some empty volume to play around. I purchased the 2 mm diameter balls from aliexpress on this LINK. 

About the clear window pieces, there are two options: 

Balls are placed in between the two clear windows, kept laterally by the separator piece: 

Assembly basically is bottom up:

1. Slightly glue the backframe to bottom clear window, just enough to keep in place.
2. Slightly glue the separator to bottom clear window, just enough to keep in place. 
3. Place the 2000 balls inside the separator. Slowly! they jump like monkeys.
4. Place the top clear window and secure by bolts. 

On step 3: If too fast those tiny balls will be never found even if you live to see two more sun full eclipses. Use a flat bowl or similar to easily collect missing balls. 

Metal hardware you would need: 

IF making the clear windows on transparent PETG I recommend following the indications given on this link:

The heat treatment effect (grain growth) can be induced by hand shaking the frame:

…or by vibrations from a speaker. I used this android app to find resonating frequency:

The Steve Mould video that inspired me: LINK