They are NOT illigal, I have had them on my last three cars and have never been pulled over. As for numbers, according to my neighbour who is a boy in blue, there is no actualy law stoping you from putting numbers on but if you have them on and are driving like a dik, look out they will get you on something (this may be true or not).
As for cutting them out yourself why go throug the hassle? if they are not the right size they look like crap, and cutting them is crap. nothing worse than a home made job. No only that contact from the local shop is not the same sort of vinyl and does not stick.
TIP, spray the area where you are going to attach the disk with soapy water. Then romove the disk completeley from the backing paper and spray with soapy water. Place disk on car close the right spot and slide into place - no need to rush and you can remove them and start again in they are really crroked. One in place get a squeegie and work from the centre out removing all air bubble sand water. Leave to dry.
As you can see from two of the attched attached pix I had no trouble driving around with numbers on though there were times where I would put a slash through them. I got an email the other day from teh guy who bought my green mini and he still has #8 on it.
http://www.shoalhaven.net.au/~motorspor ... 2/p023.jpg
http://www.shoalhaven.net.au/~motorspor ... 2/p053.jpg
http://www.shoalhaven.net.au/~motorspor ... arm014.jpg
http://www.shoalhaven.net.au/~motorspor ... P_0007.jpg