Not sure if this helps, but I just saw in bunnings yesterday they have started stocking core flute in 2400mm x 1200mm sheets.
edit. I just looked on their web site and cannot find it, but it was around 10mm thick and may be a temporary line so you may need to go in and see if your local shop has any.
Otherwise last time I wanted some I went to the local sign writers and asked if I could buy some off them and got 5 sheets.
Not super cheap but i have used 8mm polycarbonate in dark grey . Easy to cut and drill. Available at Bunnings. Matrix in photo is 2x 1200x600x 8mm panels. Also the spinners/circles are made from the same material