I'm sick and tired of either ending up with a pimple on the roof, or another cracked unit that was actually good.
Thusly I purchased this:
![Image](https://i.ebayimg.com/images/g/gJwAAOSwKSlb~fIu/s-l500.jpg)
They are hollow drills and work a treat. You drill the plastic around the rivet and the insert will fall out without the infamous "snap" sound.
I bought a 6mm and an 8mm drill, they cost me less than two quid including postage.
One window unit saved = £2.80 plus postage, so using them only once has paid for them more than once.