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Hi Shep, I'm not a mason but have built a few small kilns. Firebrick can handle the high heat with fire clay as mortar. The 2x4x8 or 1x4x8 firebrick I got at the brick yard. There fairly soft so you may need to line the outside with regular brick or concrete. I used a skillsaw with a masonary fiberglass blade to cut with and a mason bit to drill holes. They drill and cut very easy but very dusty. I like 30", ID round x 5ft high (short wife), a 6ft burns considerable hotter, for a fire box as you get a hotter flame due too the increased stack air draw (forced draft) vs square. I found that 3 equal spaced vents around the bottom 8"x16" bottom vents works well with one open to about 20" for easy ash clean out. No.6 or No.8 rebar spaced on 3" centers works well for a grate with a layer of fireclay on each rod for iron protection coating. Grate sets above vent opening. Each number of rebar equals 1/8" so a No.6 rebar is 3/4" round. With this design you get a extreamly hot burn that doesn't smoke too bad thus less trash smell and will burn until nothing but ash is left in the bottom. I've seen flames 6ft above the top if I feed it to fast. Once the hot burn is done with then very little fly ash comes out of the top so I don't use a top spark screen even with card board but I also don't leave it unattended until the fire dies down below the top. I do pre-screen my trash for metals, two trash cans in the house. I can burn 72cft of trash in less than one hour with a 10mph wind. If the trash is slightly wet I add some small (1" diameter) sticks. T_Bone
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Rod