nomma, here are some molds I made using silicon and then alumalite casting resin. Both products came in a kit purchased at Toledo last year. The molding compound is pretty easy to use. You need to make a mold "box" and then add the silicon. What you may want to consider is molding half a cylinder at a time. You want the separation line on the center of the cylinder. I would place the cylinder half fin side up on a flat surface. I use a piece of glass, then pour the silicon. Here are a few parts I made for a manifold. I needed 8 of them so I made one from styrene and copied it via the molding process.
The last picture is of the pieces added to the manifold.