I think if you did the wool under firebricks, you will be fine. You can get 55 lb. bags of refractory mortar (Satanite) for $88 and that should be enough to do everything. I would lay down the wool, do a wash coat of satanite and let it dry. Then lay down the mortar like you were laying a brick wall, small areas at a time. Once you get all the brick layed in, let it dry overnight and then maybe get a fire in it to make sure everything cured under the bricks. I have been wanting to do this also, i have 500 lbs. of fire brick in my shop for this project. I think that with that much brick and mortar/insulation, you could build this on a wood frame and it should even get warm. I built a small forge for fun out of bricks, and at welding temps i could still touch the outsides of it.