Well if you really want to learn something:
Metal Casting Part-1
Metal Casting Part-2
Or do what NASCAR does—use a restrictor plate, with the same manifold and carburetor. For a highly engineered product: the
Gene Berg Jet-A-Vator.
Else just grab an aluminum or phenolic plate and a drill and have at it. NASCAR uses one hole per port. I can't find a good picture of the Berg product, but it has ~1/8" holes in 60-80% of the port area. This gives some additional benefit, anti-reversion and better atomization.