Not sure how you could fashion it with metal. If possible, I'd try building it like a hoop coop door. Some examples and links below may give you some ideas.
IF you wanted to do metal, you could probably find galvanized fencing tubing at Lowes/HD, cut and use fittings to extend/expand the door to fit more snuggly, but you'd have to keep in mind that settling may occur (with any door), so you'd want a way to adjust fit as needed.
Are you going to cover the structure/pen with hardware cloth? While the framework of those do seem pretty good, I've seen quite a few folks using them, the wire that is also included is thin chicken wire which can easily break or be torn, so covering with a sturdier wire would be something to consider doing as well.
You may find that just building your own hoop coop may be just as easy or use that framework as a starter and add some cattle panels to make it sturdier. A lot would depend on your climate and the location of the pen/coop too. If you have a lot of snow/ice or wind, etc., you want the structure as solid as you can make it, this way it will last for years to come.
https://www.backyardchickens.com/articles/mos-cattle-panel-hoop-coops.73385/
https://www.backyardchickens.com/articles/aarts-hoop-coop-chicken-tractor.72211/
