Simple quick comparison for sandbox base and thickness
First pic is 3mm door skin
Second is 5.2 mm door skin
Last is 4.5mm extruder acrylic ( ask me how I now know it’s extruded , reference the crazed background lol must be mindful of expansion and hole diameter)
I just grabbed a piece of scrap Luan that was mostly flattish and cut off the parts that had black spots.
It was 1/8ishy based on my high precision thumb and finger gauge.
As for me … pretty much the same 
Hope that helps!
I used 3mm base plate and just stacked enough magnet to grab and move the ball like I wanted 
Late to the party, but have some useful input here…
I originally built Arrakis with 1/4" Baltic birch plywood bottom, used a 1" cube N52 magnet, and left a 2-3mm air gap between the magnet and the bottom of the board so it wouldn’t make any noise. I found that the plywood would warp with changes in humidity and the air gap would disappear and the magnet would make noise as it dragged on the bottom of the table.
For Arrakis 2.0 I tested 5mm polycarbonate but found it sagged too much (Arrakis is a large table). I solved the problem by using a piece of 5mm thick glass for the bottom of the table. Minimal sag, and absolutely doesn’t warp with changes in humidity.
