The one above (on the casting page) worked very well, and I had made another one using some sandy vermiculite-filled clay I had laying around, but because the whole thing never got fired, it remained soft and broke up on me. :( So I'm building a new one, and since I don't have any more clay to brew my own refractory, I'm trying some commercial stuff instead! 8-)
Here's the diagram, more or less, of the intended product. Yes it will have a lid section: it will have a concavity right above the hearth, and two side projections (similar to the burner and pouring spout "gouges", but taller) for exhaust.
Here's the form, just about ready. Just going to duct tape up the holes, then I can mix up a batch of stuff...
This is the pink (I repeat, damn camera) foam pattern which will form the hearth cavity. It's just hot-wire cut pink house insulation foam, hot-glued together.
Here's the stuff: Plibrico brand Plicast LWI-26, 50lbs. bag. Payed through the teeth for it too, $30 for that thing. They say it's because of shipping from where it's made, oh well. Instructions on it say 6 1/4 quarts of water per bag, and even gives a Precise Scientific Test you can perform on it: "grab a handful, throw it 1 foot in the air, if it crumbles when cought it is too dry, if it sags through the fingers it is too wet." But hey, it makes sense!
Here's the tub containing about 500 cu. in. of the stuff, all mixed up. It has a coarse texture, probably perlite, combined with a fine dust of refractory cement. That claw digging tool works wonders mixing refractory!
Here's the furnace, all finished and setting up. It goes in more like cement than molding sand; you don't compress it in, you vibrate and push it. Because of the coarseness it's a cross between the two, but I'd still say it's more like cement. Instructions say to let this cure for at least one day before firing, I'll give it two then put it in the oven. In the mean time I'll do the lid...