I picked up a few 6" x 4" picture frame in Asda (Wallmart) the other day. They're broad based, so stand up on their own.
The LED matrix is the perfect size. I used a bit of backing board to hold everything in place - a bit on the top and bottom of the matrix so it sits in the frame and a sheet on top of that which the arduino and circuit board are attached to.
I made a small shield using strip board to make it easier to remove the arduino and plug it all back in again easily. I also used strip board to hold the RTC, relay (with switch, it's quite annoying so I can turn it off easily) and connection for the temperature probe (the latter could've gone on the shield, but I didn't decide to do that til the end).
The clock shows hours, minutes, seconds. Every other time the seconds end in a 2 it shows either the day of the week or the temperatre.
I've got a picture of the arc du triomphe in the frame. When the clock is off it looks like a normal frame, when it's on the display is clearly visible.
This is the back:
This is it on my fireplace: