What do you mean "design"? It sounds like you just need a basic rectangular box. No need for much elabotare 2 x 4 framing; a four sided plywood base, with a couple of cleats to secure it to the floor. Be sure it is square before securing to floor, then add a double thickness of plywood for the top, some backer board, and tile. Have an idea for your mantle, so you can make the box wide enough to receive the vertical legs comfortably.
Although it will certainly cost more than marble tiles, I always think that slab stone look better on a fireplace than tiles., but that is MY opinion, and the only one that really matters is the one of the person who is paying for it.
Last edited by troubleseeker; 02-18-2008 at 07:30 PM.