I have done a few. Its been, umm, 25 years. ug. Yours is an inset style meaning it fits between the bricks, not an overlay style which goes over the bricks. An inset style is more of a pain because you have fewer options. We used to make templates in the field. I was the kid who had the privilege of getting inside the fire box while someone held up a big piece of cardboard over the opening. I took a marker and drew the opening from the inside.
We sent these templates to the manufacturer who fabricated the door. The install was easy, just a couple of brackets and masonry anchors.
Portland Willamette was one brand that could be done custom as I recall.
Portland Willamette :
For you, you likely do not need a template as things look square and straight. The template was for arched or irregular stone fireplaces.