See this
http://www.biabrewer.info/viewtopic.php?f=53&t=2052
OR
Trace your kettle lid, cut circle slightly larger, short side of rectangle comes from kettle height plus how much you wish to overhang the top rim when doughing in, cut a long rectangle of voile a few cm longer than the circumference of your lid, sew the circle edge to the long edge with half inch (1.25 cm) seam all around. Sew vertical edges with same seam width, trim excess from the rectangle at the end of the circle. Hold sewn seam pieces taut and approach a candle flame carefully to melt hairs together.
My wife sewed the circle style in voile for me. I'm lucky. 24 brews later I pinched a hole in the bag, sewed it shut and it still works. Cinching a cord around the opening lifts 7 kg (14+ lbs) with no problems. There is a video on YouTube showing a voile bag holding more than 80 kg. See brew bag test.avi there
