its just a lath with a point on
if you get some board alittle smaller that what you need to scribe it and place it in the opening then get the lath and place it point first onto the wall and draw round the lath onto the board,
keep drawing round the lath every inch or so all the way round.
then place the board over the material you want to fit in the opening then if you hold the lath over the marks you made whilst drawig round, then mark the point on the new material and you end up with a dot to dot to cut to a perfect fit
if that makes sence......
edit found some pics and better explination
http://www.woodworkuk.co.uk/forum/viewtopic.php?f=9&t=5704&p=72099