I had the same question, but I figured I'd try it out with 1/4" - 3/8" stock.
I just assembled the X-stage of a McWire using a $2 8.5X11 cutting board from WalMart that is just under a quarter inch thick (probably right around 6mm). These seem to be working ok, you just have to be careful that the countersink doesn't go too far or it'll start to go through and widen up the 5/16 hole too much.
I can post pics later if needed.
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Nik