Using William Wynne's design as a starting point, I decided to try to make my own intake manifold. I went to a muffler shop, bought a piece of 1.5 inch exhaust pipe, and had them make several bends in it using their hydraulic bending machine. I had them make two 90-degree bends and two 60-degree bends, leaving about a foot between each bend. Then I took it home and cut the bends apart, and trimmed each one to size so that it would fit the way I wanted it to.
It turns out that it's a good thing I didn't pay $300 for William Wynne's, because it wouldn't have fit! I had to raise the horizontal portion by about 3 inches in order to clear the nose gear mount. As a result, the carburetor down-tube ended up being a bit longer. The down-tube is actually made from 2-inch exhaust pipe. (I first tried using some 1.5 inch, but it interfered with the carburetor's butterfly valve.) I still need to weld some tabs to the horizontal piece in order to more securely attach it to the engine, but that will be easier with the engine removed.
Total Time: 12.5 hrs.