I wanted to build some cabinets to his our computer, stereo, game consoles, and seeing machines. I needed something that would allow a remote control to work through, so I used decorative metal sheeting found at the orange hardware store. They're built to be nearly flush with the fireplace. The left cabinet is made of 1x8s and measures 36"x36". There are 2 interior shelves. The shelves are slightly shorter cuts, measuring 34 1/2. The right cabinet is the same as the left, but with a 18"x36" unit kreg jigged to the front. That was to accommodate the larger media units while keeping the cabinet flush with the fireplace.
The fronts are just 2" trim frames with metal sheeting screwed to the inside. I used metal corner braces to hold the corners in place. They're mitered corners. The hinges are cheap $2 hinges and magnetic closures hold the doors closed.
I also made the mantle. It's a piece of 1x12 on top with a flat foam piece of molding and 2 corner pieces.