For outside ?
your going to need some kind of galvanised wire, but you will need to check on how to prepare it for birds, I’ve read a few wash in vinegar and leave a week or two before putting birds in but others here can confirm this better.
or in an ideal world stainless mesh but that’s not cheap.
if he’s going to be out for some time you also may want to think about double mesh, so an inner layer and outer layer to stop predators getting little toes and things that the bird could stick through, the outside could be any kind of mesh as it theory the bird could never touch it.
then you also have to consider the timber and how it’s treated.
ive thought about this and if it was me although expensive the Rosemead stuff is ally, so won’t rust, never needs to be treated and is just ready to use and safe.
I know others here have outside flights so I’m sure they can give better advise than me.