What a relief, I was afraid you were asking about the lampshade.

I imagine you could bolt your frame to the studs in the walls and then slide your plywood facing over the framing and attach, probably with finish nails.

Thanks all. I should have mentioned that it will be an electric fireplace. I was just curious on the bench idea.

I think this 2x6 bolted into the studs with the plywood casing should do the trick

There are a number of specialty bench bracket manufacturers, such as Original Granite Bracket and Federal that sell hidden and surface mount brackets. Small auto welding shops can also fabricate what you want in the way of hidden brackets. Considering that up to 75% of Americans are overweight/obese, strengthen adequately.

Yeah I found these brackets. I figured every other stud can do the trick if I do a 16"-18" depth.

https://www.amazon.com/gp/product/B0752SK3RQ?pf_rd_r=5PHWM55P28XWJGZBZF22&pf_rd_p=edaba0ee-c2fe-4124-9f5d-b31d6b1bfbee