David, I used 3mm thick Masonite (which I believe is called hardboard where you live) this was glued to quality plywood about 8mm thick, making the whole about 11mm thick.
The main frame is two pieces of 18mm plywood glued together to make an approximate 36mm thick circular frame. It is quite rigid, that said, the wheels are usually only skimming, except when there is a load on, and then I ensure my tools are sharp and taking light cuts. I use it for long spindle turning, where it works brilliantly.
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