You place Plexiglas or sheet metal with a rain drain under the venting. Much like the Toyota MR2 engine covers that are open to the weather also. The Toyota though did not come with a device to channel water. So, they went through alternators rather quickly. I went as far as replacing the rear engine cover to a supercharger cover that was only vented on the drivers side for the intercooler and the passenger side was not trued. Simple piece of Plexiglas solved all the water issues.

My original idea of the chevy cowl is my first choice and it would keep any type of water from pouring down. I like the style of the cowl hood also. Most likely I will just get another hood at the junk yard and work that instead of my original hood. I need to repaint the truck in the near future as well and want to do a couple changes, the hood being one of them.