Nice. I made one earlier this year to use in a vampire scene I intend on doing and yours makes mine seem so blah as usual. I wish I had the patience and skill of so many of you on this forum. Very well done.
I made a foam fireplace last year for the mansion room. It was a fun project, here's some pics. 1" thick blue styrofoam insulation and some wood crown moulding was used for materials. The base is 2" thick foam.
I used some wood to support the base, and gorilla glue to put it all together. Drylock and dry brushing on the finish. I intended to have glowing eyes that light up as part of the fireplace, but never got there...
Icemanfred, there's a free pieces of wood supporting the sides, but not a full frame. I used the denser 250 psi pink foam so it was pretty solid on its own (plus it was sale...). To get the depth, I just really ground down the edges with a random orbital sander.