Shake Table

In 8th grade science, we are studying plate tectonics and earth features, so earthquakes come up. I was having them research the idea of earthquake resistant design...and some were doing it, but most were not. I know I wanted them to build...but pandemic. After some discussions with other teachers, we decided we could do it safely. So I needed a shake table (the school has one, but the other teacher wants to do it also and I dont want to go up & down stairs to share). I built 2.

materials to make 2
2 - 3/4" pvc (10' ; 8' would work)
8 - elbows (10 pack)
16 - 3/16" x 2" eye bolts (nut included)
16 - s-hooks
rubber bands or springs
2 - 12"x12" board
marbles
round plastic ring (top from frosting can or margarine tub)

I cut the PVC into 20" pieces (8)
I drilled a 3/16" hole, 3 inches away from midpoint on either side for each piece (2 holes per piece) all the way through both sides of the pvc
I cut boards to 12"x12". I had some scrap, 3/4" mdf.
I am wondering in 1/4" would be "better"?
I wouldnt go thicker than 3/4"
I drilled 2 holes per edge, about 4 inches from center on the side, about 3/4" in from edge
I wanted the bands/springs to NOT be perpendicular to board/pvc (which is why on pvc holes are 3" from center and boards 4" from center)

To assemble
- feed the eye bolts through the holes in the 20" pvc pipes
- attach (4) 20" pieces and (4) elbows to make a "square"
- thread rubber bands through the holes in the board and loop through itself.
- if the eyebolts are facing "in" , use s-hooks to attach rubber bands to eyebolts
You can rotate the PVC so the eyebolts are up, or even pointing outwards. In this case you do not need the s-hooks
- put marbles in the plastic top, slide underneath board to the center. This keeps it off the table, counter and reduces friction. This is especially important if using rubber bands as they would be touching the table/counter.

With mine, if you pull the board sideways, then release it, it shakes well at first but dies quickly. I wonder if springs work make shaking last longer since they have a push AND a pull.
Pulling a corner diagonally, the vibrations last longer. 
They also last longer if you twist a corner, then release.
A couple of students just did the shaking manually, literally shaking the shake table.
That worked fine.