Although, I strongly believe that 'natural is best' when it comes to looking after our parrots, and wooden nest-boxes which parrots can chew to their heart's content and which have good insulative properties, are often better than manmade materials, which can tend to get too hot or cold inside, there are exceptions to every rule.
In my Coral-billed pionus article in the August issue of Parrots, innovative metal nest-boxes are used with great success, but the breeder gets round the chewing problem by covering the entrance hole with pieces of pine, so they have to chew their way in, and in addition they have pieces of pine to chew inside the nest-box too.
Even the much talked about metal surrounds to wooden nest-box extrances can be made chewable by placing pieces of pine over them, so the parrots have to chew away at the wood to gain entry. The needs of every species is different, and the time of year that they breed should also be taken into consideration, as metal certainly wouldn't work too well for a winter breeder, and plastic not so well for a mid-summer breeder.
Take a look at the article, there are loads of tips in there, which are applicable to not only other pionus species, but to all breeders of parrots.