I've been experimenting with different mixes of refractory cement and portland cement. Today, I tried a mix of 10:4:5 (refractory cement, portland cement and water) by weight.
I whipped together a mould out of polystyrene foam (seeing as I've got 'tons' of this stuff lying around now) with a couple of square inserts as an experiment to see how well the concrete will conform to the shape.
After mixing up the two cements with water, I scooped it into the mould. I found that it was a little bit too moist, so next test I'll use less water.
8 hours later: the mix set quite nicely, except it seems to have the consistancy of clay before it's been fired. It can be handled carefully but is easy to mark. I'm going to let it sit overnight in the hopes that it firms up. Tomorrow I'm going to go to a 8:6:4 mix and and see how that turns out.
On the bright side, the cement mix conformed to the mould perfectly.