Boomting, I did think it through before just randomly picking something that would be completely inappropriate!
It will be in a shaded area of the run so shouldn't be too hot & my girls don't go in their coop during the day anyway. They can if they want but they don't.
It will be attached to the wooden frame of the run so won't be unstable.
They will have wooden ramps to get up to the perches - it's about the same height as the the Eglu Cube.
The feeders & waterers are in the run so they will have access to them all the time, as they do now.
I would put nest boxes on the bottom shelf.