You should get a stone arch or an angle iron to cross windows or you can also use a stone lintel if you can find one. But it can be very hard to build these windows by yourself, so I think your should hire a window repairman.
There called headers.
If you key word search "vinyl window header" lots of sites come up.
Most come with a bracket that gets mounted to the wall then the face plate gets snaped in place.
On a rough surface like that it's hard to keep the two pieces together once the start to expand and contract. We've use stainless steel trim head screws on the top and bottom to keep them together.
Last edited by joecaption; 06-21-2012 at 05:52 AM.