It's nice to see that someone else fell down the rabbit hole of casting their own parts. A thin cross section like this is almost impossible to cast as the metal cools faster the thinner it gets. A passage all around the pattern with plenty of gating and pouring from both sides at once should work pretty good. ( That means you need a helper)..... For the fins or added detail, carve them in wood, then glue on with epoxy and use that epoxy to form a smooth fillet at the joint, and also at the edge of the pattern, Aluminum like this will shrink quite a bit around the outside, and sink in the middle, and that means you need to add a riser right at the shrink area to feed it.
But you probably know this all ready. The work going in, well its a lot, and you really need to just concentrate on doing what you love. Using the original works pretty good, just have to build it up a bit and experiment with it! I have a pair of finned breather covers for the 550, and you always get a kick in the pants at the way some guys drool when you say you cast your own parts!