Not to be negative, but you might discretely inquire of your insurance
agent what effect a self installed, self built(not UL listed) fireplace
might have on your fire policy. Maybe nothing, but it would be nice to
know in advance.

All the factory metal fireplaces are double wall.
Most of the factory jobs use interlocking castable ceramic panels
simulating firebrick, you might get some replacement panels and design
around them.