Bird box accidentally in the sun

Hi, I put a bird box up on what I thought was the east side of my house. It turns out that my house is actually on an angle and it's south east. It's only now the days are longer I can can see that the bird box is in full sun for most of the day where it was previously shaded by the neighbour. There's a live nest of starlings inside and I'm really worried they're going to overheat as temperatures rise. I thought about making some form of larger roof out of ply and attaching it to the wall above the box to provide some extra shade but I'm worried if I do anything around the box, the parents may abandon the nest. Any suggestions on what I should do?