my ZCI on mine are 3mm ply made from 2 layers of 1.5mm, with another small section of 1.5mm added to reinforce the un supported section, i have also considered making them from HIPS which is not as brittle as acrylic, im planning on trying to computer cut my next batch on a new die/plotter cutter i got for work today ether in HIPS or the thin ply.