Here are some pics from when I did this back in 2005. I used a 92 mx6 intake.You don't have to use aluminum, but it would be the best option. Steel would be a paid to work, unless you just drill it out with a hole saw.
I made mine out of t6 6061 3/4" aluminum. Number 1 and 4 runners are 1/4"off like stated before. The 3/4" adapter gave me a nice smooth transition from the intake to the head without to sharp of an angle. Just cutting a large hole to transition from oval ports of the intake to the round port of the head I think will not allow for a smooth airflow. I actually opened up the intake ports a bit and made them more round. I port matched the intake to the plate and the head to the plate then matched the gaskets also.
Honestly, if I were to do it again I would just build an intake. Costs would have been about the same, but I would have had less time into it. It was all in all fun when I did it and I wanted to do something different.
Here's my post from back in the day.
The pic's are dead because that hosting was dropped.
Here's my photobucket link.
Here are a few pics also.