James, you are thinking backward on this project. A false fireplace is not somnething for you to build. It's something that you buy and, if for no other reason, if your house burns down, your insurance should cover it.
The fireplaces are designed, engineered and built for safety because the manufacturers know that you will sue them if there is a hint of a question about their safety. For this reason, too, I would let the seller of your fireplace suggest an installer or have them install it.
If you are thinking about a decorative fireplace only, why? The useable ones today are designed and engineered and manufactured and tested for safety. One type or another will fit about anywhere. Why not consider one of these?