This clear, non-pervious plastic sheeting normally used as a painters drop sheet is ideal  for felting.  It is 2.6 metres by 3.6metres wide.  Cut it up to whatever size you need.  Use instead of netting or curtain sheer to hold your fibres in place when you felt them.  Once you have wet down and soaped your wool fibres put a layer of this plastic on top  and then continue to rub or roll your work.  Also useful for waterproofing your table.