DIY Refractory Compositions thread out at the Alloy Avenue forums has a number of recipes for refractory. I made my first furnace and a few prototype stoves using a home made fire clay based refractory. There are far too many possible variations to effectively cover in a single post, but there are a few points that are helpful.
Obviously the first point is to use a high firing clay. Adding grog also helps a great deal. Grog is basically bits of clay that have previously been fired and then broken up. Having grog in your mix, preferably with a multitude of sizes as opposed to finely sifted dust, will reduce shrinkage. Being that this previously fire clay has already shrunk it makes a great addition to a fire clay hot face.
Another thing that can be helpful is the shape and design. Given that these clays can and probably will crack, it makes sense to shape it in such a way that it can't fall in on itself or easily topple over. You can use a knife or similar to add 'dashed lines' so it will more likely crack in places you designate as opposed to wherever it feels like cracking. If you wrap a more insulating material around your hot face (insulating refractory, ceramic blanket, home made insulation) this can also be shaped and designed to hold the hot face in shape. I have even seen
people make special shaped bricks out of refractory and stack them up. any of the shrinkage or cracking would happen at the brick stage as opposed to happening in the finished product.
Given the possible temperatures of these
heaters I would avoid using sand in the hot face, particularly if you are going to be adding insulation. It will turn to glass, and this molten glass will act as a flux and slump your hot face like an ice cream cone on a hot day. While I was fairly fortunate with the first furnace I built, I only got it up to aluminum melting temperatures. As temperatures increase the chance that a home made refractory mix will fail over time increases, be it hours or years. Something made to withstand intense temperatures like
Missou Castable Refractory would be better. I have heard of guys on Alloy Avenue destroying Missou with a good oil burner when melting
cast iron, but it is supposed to be one of the best refractories available. You could use this as a hot face and make your own insulation. A lot of the furnace construction threads over there use the same principles as building a
rocket stove/RMH.
They have brought up some
rocket stove stuff over at those forums and they have very experienced people there, including people who worked in proper foundries for a living. I'm sure if you can't find the information you need through searching that the people there would be capable of giving you plenty of help and ideas.