Looking for ideas to build my own insert

Anybody build their own insert?

I have a small firebox in my masonry fireplace and am using a smokedragon insert now. I would like a more efficient insert but my choices seem to limited by the small size of the firebox. It has a 23.5" x 23.5" opening and the back is closer to 18" wide. I have looked at the Lopi Answer and similiar inserts but think a "custom" homebuilt might better suit my needs.

I'm thinking I would try to find a used door with glass as a start. I have (6) 30"x 60" sheets of 3/16" steel plate that I rescued from a dumpster at work. I also have a 30"x 30" square of 1/2" plate. These should provide the bulk of the raw materials. I am confident in my welding abilities and have all the tools and space needed to take on this project.

BUT, I do want to research this as much as I can and hopefully only build this once.

So, anybody build their own insert? Got any tips? Any web links or other reference material that would help?

I plan to draw up something in paint real quick and post it to give a base for people to tweak.

Hmmm. Never considered the insurance situation. But I wonder how this would be any different than a wood stove from yester-year thats still in service. I'm sure not every woodburner in use is UL listed.
But, I'd prefer this post to stay on the original topic, not dissappear into the insurance vortex.