Yup, I did it about 2 months before I bought a Blackstone. It was a fun project and it actually worked pretty well on my Weber gasser.
Couple things I learned:
I used a piece of, I think cold rolled steel that was something like 10 gauge (don't remember). Since I don't have a welder, I drilled and bolted the sides on. In retrospect, I would have used thicker material. This heats fast, but doesn't hold the heat.
The steel I used did not have alot of scale on it so I took my grinder and put a flat disc on it and took it down to shiny metal, then sanded it smooth. That worked great. Light pressure on the grinder so you don't dig grooves.
I should have used higher sides.
If you have a welder, it might be beneficial to weld some braces on the bottom. I do get some warpage with heating, cooling and cleaning, as you can see.
There is no grease containment system on this so it can get messy.
I experimented on height above the grate and finally settled on about 3/4 inch. It is up on ceramic BBQ briquettes and that works well, BUT I did have to trim it down about an inch all the way around so the heat has somewhere to go. Otherwise it all comes out the front and you end up with a very hot belly.
If you do this, spend extra time seasoning it. It is a much different surface than a Blackstone so a little extra time will be needed.
All in all, it was a fun project and it worked well. But it is heavy and hard to store when not in use so I ended up with a Blackstone 28 Pro. Love the Blackstone and they are starting to show up on clearance at Walmart and Lowes.....
If you do make one, post pics and how you like it!