I think that's tough to answer because most of us use in-cage nest boxes you can take out or put in. Reason being, a nice cozy hole like that quickly becomes a rabbits favorite litter box in a cage, 8/10 times. Then they try to nest there too and pee all over the kits and they chill and die or get an infection or any number of other problems. So we take out nest boxes if they're not about to give birth and then turn them on their side when the kits start hopping out because they often can't get back in otherwise.
I think there's no height in which hangers-on won't get pulled out but also wobbly babies still finding their feet can get in/out.
I also hope your weather is mild temperature wise. Metal conducts heat and cold like crazy and for that reason many animals prefer still wind outside of metal to shelter in metal. On a still day it may easily be too cold (or any sunny day, too hot) inside the box for kits.
I think there's no height in which hangers-on won't get pulled out but also wobbly babies still finding their feet can get in/out.
I also hope your weather is mild temperature wise. Metal conducts heat and cold like crazy and for that reason many animals prefer still wind outside of metal to shelter in metal. On a still day it may easily be too cold (or any sunny day, too hot) inside the box for kits.