Shakytoad,
I guess that you need to measure the existing door to search and see if there are any prefab replacements you could purchase for less than a custom job. I dunno if you want the door to appear burdensome, like a heavy gate, to act as a deterrent in appearance, or instead you want it to match existing decor but perform at a higher level of security. What does Mrs. Shakytoad have to say about it?
Inside a farmhouse, I wonder if you could have made a Dutch door, but coated with metal as are exterior doors? For inside, you could take out the glass panes, and make shutter slats for them that allow air to circulate and from the inside could be opened to look thru and shoot? Also, the Dutch style allows you to open the top to lean out and shoot, and still have the bottom for cover. Just one option...I can't believe I'm writing this; it sounds like a screen script for Gunsmoke...