Putting the screw in restrictor in the case is pretty easy.
I had a stainless steel plug I was going to use, but my drill bits did not want to drill it. The aluminum drills easy, and the plug does not need to be installed tightly, so it is easy to remove. Not to mention it will be covered in trans fluid. The steel core plug is what comes in the Transgo shift kits, but if you want to experiment with different orifice sizes, the screw in plug is easier to remove, drill, and re-install. Brass would be easy to drill also, and might be able to find in the plumbing section at the local hardware store?