It might be worth taking apart one of the commercial flame pots first to reverse engineer them (outdoors with full PPE).
Here on the forum we could suggest mixes of smokeless powder with nitrates and carbonates for colour that would probably work, but they would need to be pressed into the cases and cored to control the burn rate. The most recent quote I had for smokeless powder for pyro use was £30 / lb, so it's not cheap.
If you need a brief burst of flame, mixing up a few grams of granulated 'tableau fire' in an open tube should be relatively safe and 'do-able', but the 10 ft long jets of continuous roaring flame really need to be left to commercial manufacturers. As Arthur says, given the insurance position, the Le Maitre pots are good value.
Edited by BrightStar, 20 January 2008 - 11:04 PM.