Not often I build stuff from scratch, but I'd decided to fit the TV on the chimney breast (nice and low down, rather than neck aching height) so wanted to fit the AV kit within the fire opening.
Actually I was hoping to find a lintel in the chimney, and be able to replae it with another one higher up to allow recessing the TV into the wall, but I found there was a "flue gathering" instead and AFAIK these tend to hold up all the pre-cast flue liners, so I quickly decided to just remove the non-structural bits to widen the opening and fit a cabinet in there.
I've been building up my tools collection lately, so now have decent router, circular saw, mitre saw, planer rather than my older clunkier stuff, made a reasonable fist of it from MDF, only glue required was for the moulding on the front.
How come 18mm MDF fits precisely into an 18mm routed slot, but 6mm MDF needs the thickness planing down by about 0.75mm to fit a 6mm slot?
Anyway, it was quite a tight fit within the opening, had to get the SDS out to chase the mortar snots etc from inside the chimney to get it in (some noisy revenge) fitted temporary MDF shelves in place of the toughened glass ones that are on order.
The slate hearth was chopped down from a previous attempt at doing something with the fireplace, the mains and aerial cables (along with speaker and ethernet) will run in via the side of the chimney, all the clutter on the middle shelf will eventually sit on the hidden top shelf that's behind the TV, along with an ethernet switch, chromecast, RF remote receiver etc.