In reponse to a Usenet post, here's my brazing hearth.
The hearth contains 7 firebricks of 9" x 4½" x 1¼" in a cage of welded steel angle. It's an L-shaped hearth, rather than U-shaped, so that I can fit long items into it.
Construction is simple. The bricks are mainly held in by friction, but there's a folded tab to stop them sliding out lengthwise.
To make it, I simply clamped the bricks in place with the angle iron, then welded strip over them.
If you're buying firebricks (which are cheap, but a little hard to find) get some spares at the same time. You will also need bricks or small cubes to pile up in the hearth around your workpiece.