I did a cheap one, cost me nothing, and has a surface resistivity of 5M per square. Used rubber conveyer belting, around 5mm thick. Carbon black loaded natural rubber vulcanised to cotton plies and with an antistatic top rubber. Can be cut, but quite resistant to accidental damage, and will withstand the odd contact with hot solder or the soldering iron tip. added a corner screw and washer to connect an earthing wire, and this goes to mains earth via a 100k resistor for safety, along with the standard ELCB.