You can certainly build your own mantel surround. As a matter of fact, quite a few of our customers do this. You would purchase the standalone insert, and then your carpenter or handyman can build your mantle around that.
If you go to our website, and click on “expert advice” and then “installation guide”, you will find a diagram and a downloadable PDF document with the exact dimensions and CAD drawings accurate to 1000th of an inch.
However, we strongly recommend you don’t build anything custom until you physically have our insert in your possession so that there are no surprises!