Nesting collared doves

We have 2 collared doves trying to nest on an ariel above our window, they've been around as long as I can remember but I've been reading up about how they're not the best nest builders, and they seem to be trying to build on this ariel, and it just isn't working. They do it all day and they just fall down. Is there anything I can put out (hanging basket maybe) that they would nest in? I'm worried she will lay and it will just fall